diff --git a/modules_dart/transform/lib/src/transform/common/names.dart b/modules_dart/transform/lib/src/transform/common/names.dart
index db5e8ffe25..44c87e9ab0 100644
--- a/modules_dart/transform/lib/src/transform/common/names.dart
+++ b/modules_dart/transform/lib/src/transform/common/names.dart
@@ -84,5 +84,9 @@ String _toExtension(
           '$toExtension';
     }
   }
-  return uri;
+  throw new ArgumentError.value(
+      uri,
+      'uri',
+      'Provided value ends with an unexpected extension. '
+      'Expected extension(s): [${fromExtensions.join(', ')}].');
 }