diff --git a/src/JsonApiDotNetCore/Serialization/JsonConverters/ResourceObjectConverter.cs b/src/JsonApiDotNetCore/Serialization/JsonConverters/ResourceObjectConverter.cs
index 8bc67c0196..d20bdd5f0d 100644
--- a/src/JsonApiDotNetCore/Serialization/JsonConverters/ResourceObjectConverter.cs
+++ b/src/JsonApiDotNetCore/Serialization/JsonConverters/ResourceObjectConverter.cs
@@ -226,11 +226,11 @@ public override ResourceObject Read(ref Utf8JsonReader reader, Type typeToConver
                             }
                         }
 
-                        attributes.Add(attributeName, attributeValue);
+                        attributes[attributeName] = attributeValue;
                     }
                     else
                     {
-                        attributes.Add(attributeName, null);
+                        attributes[attributeName] = null;
                         reader.Skip();
                     }