diff --git a/source/Halibut/Portability/TypeExtensions.cs b/source/Halibut/Portability/TypeExtensions.cs
index 8a5eefebf..80f0d373e 100644
--- a/source/Halibut/Portability/TypeExtensions.cs
+++ b/source/Halibut/Portability/TypeExtensions.cs
@@ -17,5 +17,10 @@ public static bool IsValueType(this Type type)
         {
             return type.IsValueType;
         }
+
+        public static bool IsNullable(this Type type)
+        {
+            return !type.IsValueType() || Nullable.GetUnderlyingType(type) != null;
+        }
     }
 }
diff --git a/source/Halibut/ServiceModel/ServiceInvoker.cs b/source/Halibut/ServiceModel/ServiceInvoker.cs
index 29e6f3556..87339b153 100644
--- a/source/Halibut/ServiceModel/ServiceInvoker.cs
+++ b/source/Halibut/ServiceModel/ServiceInvoker.cs
@@ -68,7 +68,7 @@ static MethodInfo SelectAsyncMethod(IList<MethodInfo> methods, RequestMessage re
                 {
                     var paramType = parameters[i].ParameterType;
                     var argType = argumentTypes[i];
-                    if (argType == null && paramType.IsValueType())
+                    if (argType == null && !paramType.IsNullable())
                     {
                         isMatch = false;
                         break;