diff --git a/packages/@aws-cdk/aws-appsync/lib/schema-intermediate.ts b/packages/@aws-cdk/aws-appsync/lib/schema-intermediate.ts
index 6236a00ce1803..a661d4e4baf61 100644
--- a/packages/@aws-cdk/aws-appsync/lib/schema-intermediate.ts
+++ b/packages/@aws-cdk/aws-appsync/lib/schema-intermediate.ts
@@ -381,7 +381,7 @@ export class UnionType implements IIntermediateType {
     if (options.field && !(options.field.intermediateType instanceof ObjectType)) {
       throw new Error('Fields for Union Types must be Object Types.');
     }
-    this.definition[options.field?.toString() + 'id'] = options.field;
+    this.definition[options.field.toString()] = options.field;
   }
 
   /**