diff --git a/src/JsonApiDotNetCore/Resources/Annotations/AttrAttribute.cs b/src/JsonApiDotNetCore/Resources/Annotations/AttrAttribute.cs
index d31415df57..e1cd0a0f61 100644
--- a/src/JsonApiDotNetCore/Resources/Annotations/AttrAttribute.cs
+++ b/src/JsonApiDotNetCore/Resources/Annotations/AttrAttribute.cs
@@ -6,7 +6,7 @@ namespace JsonApiDotNetCore.Resources.Annotations
     /// Used to expose a property on a resource class as a JSON:API attribute (https://jsonapi.org/format/#document-resource-object-attributes).
     /// </summary>
     [AttributeUsage(AttributeTargets.Property)]
-    public sealed class AttrAttribute : ResourceFieldAttribute
+    public class AttrAttribute : ResourceFieldAttribute
     {
         private AttrCapabilities? _capabilities;
 
diff --git a/src/JsonApiDotNetCore/Serialization/BaseDeserializer.cs b/src/JsonApiDotNetCore/Serialization/BaseDeserializer.cs
index 67e4f41c71..1c2e85d39c 100644
--- a/src/JsonApiDotNetCore/Serialization/BaseDeserializer.cs
+++ b/src/JsonApiDotNetCore/Serialization/BaseDeserializer.cs
@@ -70,7 +70,7 @@ protected object DeserializeBody(string body)
         /// <param name="resource">The parsed resource.</param>
         /// <param name="attributeValues">Attributes and their values, as in the serialized content.</param>
         /// <param name="attributes">Exposed attributes for <paramref name="resource"/>.</param>
-        protected IIdentifiable SetAttributes(IIdentifiable resource, IDictionary<string, object> attributeValues, IReadOnlyCollection<AttrAttribute> attributes)
+        protected virtual IIdentifiable SetAttributes(IIdentifiable resource, IDictionary<string, object> attributeValues, IReadOnlyCollection<AttrAttribute> attributes)
         {
             if (resource == null) throw new ArgumentNullException(nameof(resource));
             if (attributes == null) throw new ArgumentNullException(nameof(attributes));