diff --git a/Dapper.Contrib/SqlMapperExtensions.cs b/Dapper.Contrib/SqlMapperExtensions.cs
index 7aac1004..f8a8f216 100644
--- a/Dapper.Contrib/SqlMapperExtensions.cs
+++ b/Dapper.Contrib/SqlMapperExtensions.cs
@@ -153,7 +153,7 @@ namespace Dapper.Contrib.Extensions
             var explicitKeys = ExplicitKeyPropertiesCache(type);
             var keyCount = keys.Count + explicitKeys.Count;
             if (keyCount > 1)
-                throw new DataException($"{method}<T> only supports an entity with a single [Key] or [ExplicitKey] property");
+                throw new DataException($"{method}<T> only supports an entity with a single [Key] or [ExplicitKey] property. [Key] Count: {keys.Count}, [ExplicitKey] Count: {explicitKeys.Count}");
             if (keyCount == 0)
                 throw new DataException($"{method}<T> only supports an entity with a [Key] or an [ExplicitKey] property");