diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 77eb4fb690..a35c0c5c84 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -166,10 +166,9 @@ function flattenBehaviors(behaviors, list, exclude) {
 function mergeProperties(target, source) {
   for (const p in source) {
     const targetInfo = target[p];
-    const value = targetInfo && targetInfo.value;
     const sourceInfo = source[p];
-    if (sourceInfo.value === undefined && value !== undefined) {
-      target[p] = Object.assign({value}, sourceInfo);
+    if (!('value' in sourceInfo) && targetInfo && ('value' in targetInfo)) {
+      target[p] = Object.assign({value: targetInfo.value}, sourceInfo);
     } else {
       target[p] = sourceInfo;
     }