diff --git a/src/Microsoft.ML.Data/Data/SchemaDefinition.cs b/src/Microsoft.ML.Data/Data/SchemaDefinition.cs
index 157d35d940..b26cd9554c 100644
--- a/src/Microsoft.ML.Data/Data/SchemaDefinition.cs
+++ b/src/Microsoft.ML.Data/Data/SchemaDefinition.cs
@@ -28,7 +28,7 @@ public sealed class KeyTypeAttribute : Attribute
         /// </remarks>
         public KeyTypeAttribute()
         {
-
+            throw Contracts.ExceptNotSupp("Using KeyType without the Count parameter is not supported");
         }
 
         /// <summary>