diff --git a/lib/legacy/legacy-data-mixin.js b/lib/legacy/legacy-data-mixin.js
index 018ccd31f9..a2443c7926 100644
--- a/lib/legacy/legacy-data-mixin.js
+++ b/lib/legacy/legacy-data-mixin.js
@@ -30,7 +30,7 @@ const UndefinedArgumentError = class extends Error {
  * 
  * @param {Object=} effect Effect metadata object
  * @param {Object=} fnName Name of user function, if known
- * @return {Object=} Effect metadata object
+ * @return {?Object} Effect metadata object
  */
 function wrapEffect(effect, fnName) {
   if (effect && effect.fn) {