diff --git a/src/OpenIddict.Server/Internal/OpenIddictServerProvider.cs b/src/OpenIddict.Server/Internal/OpenIddictServerProvider.cs
index 5db8fcbf..0a3ae921 100644
--- a/src/OpenIddict.Server/Internal/OpenIddictServerProvider.cs
+++ b/src/OpenIddict.Server/Internal/OpenIddictServerProvider.cs
@@ -4,9 +4,11 @@
  * the license and the contributors participating to this project.
  */
 
+using System;
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
+using System.Text;
 using System.Threading.Tasks;
 using AspNet.Security.OpenIdConnect.Extensions;
 using AspNet.Security.OpenIdConnect.Primitives;
@@ -75,6 +77,20 @@ namespace OpenIddict.Server
                          context.Request.IsTokenRequest(),
                 "The request should be an authorization or token request.");
 
+            // While null/unauthenticated identities can be validly represented and are allowed by
+            // the OpenID Connect server handler, this most likely indicates that the developer
+            // has not correctly set the authentication type associated with the claims identity,
+            // which may later cause issues when validating opaque access tokens, as the resulting
+            // principal would be considered unauthenticated by the ASP.NET Core authorization stack.
+            if (context.Ticket.Principal.Identity == null || !context.Ticket.Principal.Identity.IsAuthenticated)
+            {
+                throw new InvalidOperationException(new StringBuilder()
+                    .AppendLine("The specified principal doesn't contain a valid or authenticated identity.")
+                    .Append("Make sure that both 'ClaimsPrincipal.Identity' and 'ClaimsPrincipal.Identity.AuthenticationType' ")
+                    .Append("are not null and that 'ClaimsPrincipal.Identity.IsAuthenticated' returns 'true'.")
+                    .ToString());
+            }
+
             if (context.Request.IsTokenRequest() && (context.Request.IsAuthorizationCodeGrantType() ||
                                                      context.Request.IsRefreshTokenGrantType()))
             {
diff --git a/test/OpenIddict.Server.Tests/Internal/OpenIddictServerProviderTests.cs b/test/OpenIddict.Server.Tests/Internal/OpenIddictServerProviderTests.cs
index 4f734af8..b4c5bb03 100644
--- a/test/OpenIddict.Server.Tests/Internal/OpenIddictServerProviderTests.cs
+++ b/test/OpenIddict.Server.Tests/Internal/OpenIddictServerProviderTests.cs
@@ -9,6 +9,7 @@ using System.Collections.Immutable;
 using System.Linq;
 using System.Reflection;
 using System.Security.Claims;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using AspNet.Security.OpenIdConnect.Client;
@@ -114,6 +115,33 @@ namespace OpenIddict.Server.Tests
             Assert.Equal("value", (string) response["custom_string_parameter"]);
         }
 
+        [Fact]
+        public async Task ProcessSigninResponse_ThrowsAnExceptionForInvalidIdentity()
+        {
+            // Arrange
+            var server = CreateAuthorizationServer();
+
+            var client = new OpenIdConnectClient(server.CreateClient());
+
+            // Act and assert
+            var exception = await Assert.ThrowsAsync<InvalidOperationException>(delegate
+            {
+                return client.PostAsync(TokenEndpoint, new OpenIdConnectRequest
+                {
+                    GrantType = OpenIdConnectConstants.GrantTypes.Password,
+                    Username = "johndoe",
+                    Password = "A3ddj3w",
+                    ["use-null-authentication-type"] = true
+                });
+            });
+
+            Assert.Equal(new StringBuilder()
+                .AppendLine("The specified principal doesn't contain a valid or authenticated identity.")
+                .Append("Make sure that both 'ClaimsPrincipal.Identity' and 'ClaimsPrincipal.Identity.AuthenticationType' ")
+                .Append("are not null and that 'ClaimsPrincipal.Identity.IsAuthenticated' returns 'true'.")
+                .ToString(), exception.Message);
+        }
+
         [Fact]
         public async Task ProcessSigninResponse_AuthenticationPropertiesAreAutomaticallyRestored()
         {
@@ -1471,7 +1499,10 @@ namespace OpenIddict.Server.Tests
                         return Task.CompletedTask;
                     }
 
-                    var identity = new ClaimsIdentity(OpenIddictServerDefaults.AuthenticationScheme);
+                    var identity = !request.HasParameter("use-null-authentication-type") ?
+                        new ClaimsIdentity(OpenIddictServerDefaults.AuthenticationScheme) :
+                        new ClaimsIdentity();
+
                     identity.AddClaim(OpenIdConnectConstants.Claims.Subject, "Bob le Magnifique");
 
                     var ticket = new AuthenticationTicket(