diff --git a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddict.EntityFrameworkCore.Models.csproj b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddict.EntityFrameworkCore.Models.csproj
index 34675b6b..deb718e4 100644
--- a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddict.EntityFrameworkCore.Models.csproj
+++ b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddict.EntityFrameworkCore.Models.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <PropertyGroup>
diff --git a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreApplication.cs b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreApplication.cs
index 3188a9ec..5407240e 100644
--- a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreApplication.cs
+++ b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreApplication.cs
@@ -7,6 +7,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace OpenIddict.EntityFrameworkCore.Models
 {
@@ -34,7 +35,10 @@ namespace OpenIddict.EntityFrameworkCore.Models
     /// Represents an OpenIddict application.
     /// </summary>
     [DebuggerDisplay("Id = {Id.ToString(),nq} ; ClientId = {ClientId,nq} ; Type = {Type,nq}")]
-    public class OpenIddictEntityFrameworkCoreApplication<TKey, TAuthorization, TToken> where TKey : IEquatable<TKey>
+    public class OpenIddictEntityFrameworkCoreApplication<TKey, TAuthorization, TToken>
+        where TKey : IEquatable<TKey>
+        where TAuthorization : class
+        where TToken : class
     {
         /// <summary>
         /// Gets the list of the authorizations associated with this application.
@@ -42,77 +46,74 @@ namespace OpenIddict.EntityFrameworkCore.Models
         public virtual ICollection<TAuthorization> Authorizations { get; } = new HashSet<TAuthorization>();
 
         /// <summary>
-        /// Gets or sets the client identifier
-        /// associated with the current application.
+        /// Gets or sets the client identifier associated with the current application.
         /// </summary>
-        public virtual string ClientId { get; set; }
+        public virtual string? ClientId { get; set; }
 
         /// <summary>
         /// Gets or sets the client secret associated with the current application.
         /// Note: depending on the application manager used to create this instance,
         /// this property may be hashed or encrypted for security reasons.
         /// </summary>
-        public virtual string ClientSecret { get; set; }
+        public virtual string? ClientSecret { get; set; }
 
         /// <summary>
         /// Gets or sets the concurrency token.
         /// </summary>
-        public virtual string ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
+        public virtual string? ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
 
         /// <summary>
-        /// Gets or sets the consent type
-        /// associated with the current application.
+        /// Gets or sets the consent type associated with the current application.
         /// </summary>
-        public virtual string ConsentType { get; set; }
+        public virtual string? ConsentType { get; set; }
 
         /// <summary>
-        /// Gets or sets the display name
-        /// associated with the current application.
+        /// Gets or sets the display name associated with the current application.
         /// </summary>
-        public virtual string DisplayName { get; set; }
+        public virtual string? DisplayName { get; set; }
 
         /// <summary>
         /// Gets or sets the localized display names
         /// associated with the current application,
         /// serialized as a JSON object.
         /// </summary>
-        public virtual string DisplayNames { get; set; }
+        public virtual string? DisplayNames { get; set; }
 
         /// <summary>
-        /// Gets or sets the unique identifier
-        /// associated with the current application.
+        /// Gets or sets the unique identifier associated with the current application.
         /// </summary>
-        public virtual TKey Id { get; set; }
+        [AllowNull, MaybeNull]
+        public virtual TKey Id { get; set; } = default!;
 
         /// <summary>
         /// Gets or sets the permissions associated with the
         /// current application, serialized as a JSON array.
         /// </summary>
-        public virtual string Permissions { get; set; }
+        public virtual string? Permissions { get; set; }
 
         /// <summary>
         /// Gets or sets the logout callback URLs associated with
         /// the current application, serialized as a JSON array.
         /// </summary>
-        public virtual string PostLogoutRedirectUris { get; set; }
+        public virtual string? PostLogoutRedirectUris { get; set; }
 
         /// <summary>
         /// Gets or sets the additional properties serialized as a JSON object,
         /// or <c>null</c> if no bag was associated with the current application.
         /// </summary>
-        public virtual string Properties { get; set; }
+        public virtual string? Properties { get; set; }
 
         /// <summary>
         /// Gets or sets the callback URLs associated with the
         /// current application, serialized as a JSON array.
         /// </summary>
-        public virtual string RedirectUris { get; set; }
+        public virtual string? RedirectUris { get; set; }
 
         /// <summary>
         /// Gets or sets the requirements associated with the
         /// current application, serialized as a JSON array.
         /// </summary>
-        public virtual string Requirements { get; set; }
+        public virtual string? Requirements { get; set; }
 
         /// <summary>
         /// Gets the list of the tokens associated with this application.
@@ -120,9 +121,8 @@ namespace OpenIddict.EntityFrameworkCore.Models
         public virtual ICollection<TToken> Tokens { get; } = new HashSet<TToken>();
 
         /// <summary>
-        /// Gets or sets the application type
-        /// associated with the current application.
+        /// Gets or sets the application type associated with the current application.
         /// </summary>
-        public virtual string Type { get; set; }
+        public virtual string? Type { get; set; }
     }
 }
\ No newline at end of file
diff --git a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreAuthorization.cs b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreAuthorization.cs
index 6e09f432..697ede91 100644
--- a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreAuthorization.cs
+++ b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreAuthorization.cs
@@ -7,6 +7,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace OpenIddict.EntityFrameworkCore.Models
 {
@@ -34,45 +35,48 @@ namespace OpenIddict.EntityFrameworkCore.Models
     /// Represents an OpenIddict authorization.
     /// </summary>
     [DebuggerDisplay("Id = {Id.ToString(),nq} ; Subject = {Subject,nq} ; Type = {Type,nq} ; Status = {Status,nq}")]
-    public class OpenIddictEntityFrameworkCoreAuthorization<TKey, TApplication, TToken> where TKey : IEquatable<TKey>
+    public class OpenIddictEntityFrameworkCoreAuthorization<TKey, TApplication, TToken>
+        where TKey : IEquatable<TKey>
+        where TApplication : class
+        where TToken : class
     {
         /// <summary>
         /// Gets or sets the application associated with the current authorization.
         /// </summary>
-        public virtual TApplication Application { get; set; }
+        public virtual TApplication? Application { get; set; }
 
         /// <summary>
         /// Gets or sets the concurrency token.
         /// </summary>
-        public virtual string ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
+        public virtual string? ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
 
         /// <summary>
-        /// Gets or sets the unique identifier
-        /// associated with the current authorization.
+        /// Gets or sets the unique identifier associated with the current authorization.
         /// </summary>
-        public virtual TKey Id { get; set; }
+        [AllowNull, MaybeNull]
+        public virtual TKey Id { get; set; } = default!;
 
         /// <summary>
         /// Gets or sets the additional properties serialized as a JSON object,
         /// or <c>null</c> if no bag was associated with the current authorization.
         /// </summary>
-        public virtual string Properties { get; set; }
+        public virtual string? Properties { get; set; }
 
         /// <summary>
         /// Gets or sets the scopes associated with the current
         /// authorization, serialized as a JSON array.
         /// </summary>
-        public virtual string Scopes { get; set; }
+        public virtual string? Scopes { get; set; }
 
         /// <summary>
         /// Gets or sets the status of the current authorization.
         /// </summary>
-        public virtual string Status { get; set; }
+        public virtual string? Status { get; set; }
 
         /// <summary>
         /// Gets or sets the subject associated with the current authorization.
         /// </summary>
-        public virtual string Subject { get; set; }
+        public virtual string? Subject { get; set; }
 
         /// <summary>
         /// Gets the list of tokens associated with the current authorization.
@@ -82,6 +86,6 @@ namespace OpenIddict.EntityFrameworkCore.Models
         /// <summary>
         /// Gets or sets the type of the current authorization.
         /// </summary>
-        public virtual string Type { get; set; }
+        public virtual string? Type { get; set; }
     }
 }
diff --git a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreScope.cs b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreScope.cs
index a1e0e9ce..3c2525b6 100644
--- a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreScope.cs
+++ b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreScope.cs
@@ -6,6 +6,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace OpenIddict.EntityFrameworkCore.Models
 {
@@ -30,55 +31,52 @@ namespace OpenIddict.EntityFrameworkCore.Models
         /// <summary>
         /// Gets or sets the concurrency token.
         /// </summary>
-        public virtual string ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
+        public virtual string? ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
 
         /// <summary>
-        /// Gets or sets the public description
-        /// associated with the current scope.
+        /// Gets or sets the public description associated with the current scope.
         /// </summary>
-        public virtual string Description { get; set; }
+        public virtual string? Description { get; set; }
 
         /// <summary>
         /// Gets or sets the localized public descriptions associated
         /// with the current scope, serialized as a JSON object.
         /// </summary>
-        public virtual string Descriptions { get; set; }
+        public virtual string? Descriptions { get; set; }
 
         /// <summary>
-        /// Gets or sets the display name
-        /// associated with the current scope.
+        /// Gets or sets the display name associated with the current scope.
         /// </summary>
-        public virtual string DisplayName { get; set; }
+        public virtual string? DisplayName { get; set; }
 
         /// <summary>
         /// Gets or sets the localized display names
         /// associated with the current application,
         /// serialized as a JSON object.
         /// </summary>
-        public virtual string DisplayNames { get; set; }
+        public virtual string? DisplayNames { get; set; }
 
         /// <summary>
-        /// Gets or sets the unique identifier
-        /// associated with the current scope.
+        /// Gets or sets the unique identifier associated with the current scope.
         /// </summary>
-        public virtual TKey Id { get; set; }
+        [AllowNull, MaybeNull]
+        public virtual TKey Id { get; set; } = default!;
 
         /// <summary>
-        /// Gets or sets the unique name
-        /// associated with the current scope.
+        /// Gets or sets the unique name associated with the current scope.
         /// </summary>
-        public virtual string Name { get; set; }
+        public virtual string? Name { get; set; }
 
         /// <summary>
         /// Gets or sets the additional properties serialized as a JSON object,
         /// or <c>null</c> if no bag was associated with the current scope.
         /// </summary>
-        public virtual string Properties { get; set; }
+        public virtual string? Properties { get; set; }
 
         /// <summary>
         /// Gets or sets the resources associated with the
         /// current scope, serialized as a JSON array.
         /// </summary>
-        public virtual string Resources { get; set; }
+        public virtual string? Resources { get; set; }
     }
 }
diff --git a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreToken.cs b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreToken.cs
index 27114888..47d0cb5c 100644
--- a/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreToken.cs
+++ b/src/OpenIddict.EntityFrameworkCore.Models/OpenIddictEntityFrameworkCoreToken.cs
@@ -6,6 +6,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace OpenIddict.EntityFrameworkCore.Models
 {
@@ -33,53 +34,54 @@ namespace OpenIddict.EntityFrameworkCore.Models
     /// Represents an OpenIddict token.
     /// </summary>
     [DebuggerDisplay("Id = {Id.ToString(),nq} ; Subject = {Subject,nq} ; Type = {Type,nq} ; Status = {Status,nq}")]
-    public class OpenIddictEntityFrameworkCoreToken<TKey, TApplication, TAuthorization> where TKey : IEquatable<TKey>
+    public class OpenIddictEntityFrameworkCoreToken<TKey, TApplication, TAuthorization>
+        where TKey : IEquatable<TKey>
+        where TApplication : class
+        where TAuthorization : class
     {
         /// <summary>
         /// Gets or sets the application associated with the current token.
         /// </summary>
-        public virtual TApplication Application { get; set; }
+        public virtual TApplication? Application { get; set; }
 
         /// <summary>
         /// Gets or sets the authorization associated with the current token.
         /// </summary>
-        public virtual TAuthorization Authorization { get; set; }
+        public virtual TAuthorization? Authorization { get; set; }
 
         /// <summary>
         /// Gets or sets the concurrency token.
         /// </summary>
-        public virtual string ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
+        public virtual string? ConcurrencyToken { get; set; } = Guid.NewGuid().ToString();
 
         /// <summary>
-        /// Gets or sets the date on which the token
-        /// will start to be considered valid.
+        /// Gets or sets the date on which the token will start to be considered valid.
         /// </summary>
         public virtual DateTimeOffset? CreationDate { get; set; }
 
         /// <summary>
-        /// Gets or sets the date on which the token
-        /// will no longer be considered valid.
+        /// Gets or sets the date on which the token will no longer be considered valid.
         /// </summary>
         public virtual DateTimeOffset? ExpirationDate { get; set; }
 
         /// <summary>
-        /// Gets or sets the unique identifier
-        /// associated with the current token.
+        /// Gets or sets the unique identifier associated with the current token.
         /// </summary>
-        public virtual TKey Id { get; set; }
+        [AllowNull, MaybeNull]
+        public virtual TKey Id { get; set; } = default!;
 
         /// <summary>
         /// Gets or sets the payload of the current token, if applicable.
         /// Note: this property is only used for reference tokens
         /// and may be encrypted for security reasons.
         /// </summary>
-        public virtual string Payload { get; set; }
+        public virtual string? Payload { get; set; }
 
         /// <summary>
         /// Gets or sets the additional properties serialized as a JSON object,
         /// or <c>null</c> if no bag was associated with the current token.
         /// </summary>
-        public virtual string Properties { get; set; }
+        public virtual string? Properties { get; set; }
 
         /// <summary>
         /// Gets or sets the reference identifier associated
@@ -87,21 +89,21 @@ namespace OpenIddict.EntityFrameworkCore.Models
         /// Note: this property is only used for reference tokens
         /// and may be hashed or encrypted for security reasons.
         /// </summary>
-        public virtual string ReferenceId { get; set; }
+        public virtual string? ReferenceId { get; set; }
 
         /// <summary>
         /// Gets or sets the status of the current token.
         /// </summary>
-        public virtual string Status { get; set; }
+        public virtual string? Status { get; set; }
 
         /// <summary>
         /// Gets or sets the subject associated with the current token.
         /// </summary>
-        public virtual string Subject { get; set; }
+        public virtual string? Subject { get; set; }
 
         /// <summary>
         /// Gets or sets the type of the current token.
         /// </summary>
-        public virtual string Type { get; set; }
+        public virtual string? Type { get; set; }
     }
 }