diff --git a/lib/propmangle.js b/lib/propmangle.js
index 41eeffe4..f79b4748 100644
--- a/lib/propmangle.js
+++ b/lib/propmangle.js
@@ -145,7 +145,8 @@ function mangle_properties(ast, options) {
     }
 
     function should_mangle(name) {
-        return names_to_mangle.indexOf(name) >= 0;
+        return cache.props.has(name)
+            || names_to_mangle.indexOf(name) >= 0;
     }
 
     function add(name) {