diff --git a/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbAuthorization.cs b/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbAuthorization.cs
index 9a6be32b..19ef8faf 100644
--- a/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbAuthorization.cs
+++ b/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbAuthorization.cs
@@ -58,7 +58,7 @@ namespace OpenIddict.MongoDb.Models
         /// <summary>
         /// Gets or sets the subject associated with the current authorization.
         /// </summary>
-        [BsonElement("subject"), BsonRequired]
+        [BsonElement("subject"), BsonIgnoreIfDefault]
         public virtual string Subject { get; set; }
 
         /// <summary>
diff --git a/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbToken.cs b/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbToken.cs
index c99dd550..6375b119 100644
--- a/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbToken.cs
+++ b/src/OpenIddict.MongoDb.Models/OpenIddictMongoDbToken.cs
@@ -88,7 +88,7 @@ namespace OpenIddict.MongoDb.Models
         /// <summary>
         /// Gets or sets the subject associated with the current token.
         /// </summary>
-        [BsonElement("subject"), BsonRequired]
+        [BsonElement("subject"), BsonIgnoreIfDefault]
         public virtual string Subject { get; set; }
 
         /// <summary>