diff --git a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Annotations/Attributes.cs b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Annotations/Attributes.cs
index 227872199..a90a17b69 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Annotations/Attributes.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Annotations/Attributes.cs
@@ -83,7 +83,7 @@ public class SerializationConstructorAttribute : Attribute
     {
     }
 
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Enum | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Interface | AttributeTargets.Enum | AttributeTargets.Field | AttributeTargets.Property | AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = false, Inherited = true)]
     public class MessagePackFormatterAttribute : Attribute
     {
         public Type FormatterType { get; private set; }