diff --git a/src/execution/execute.js b/src/execution/execute.js
index 79a3b77062..ad630f8aac 100644
--- a/src/execution/execute.js
+++ b/src/execution/execute.js
@@ -929,7 +929,6 @@ function completeListValue(
  * null if serialization is not possible.
  */
 function completeLeafValue(returnType: GraphQLLeafType, result: mixed): mixed {
-  invariant(returnType.serialize, 'Missing serialize method on type');
   const serializedResult = returnType.serialize(result);
   if (isInvalid(serializedResult)) {
     throw new Error(