diff --git a/underscore.js b/underscore.js
index 934f3f83e..dff3daaf3 100644
--- a/underscore.js
+++ b/underscore.js
@@ -741,7 +741,7 @@
       // Ensure that both objects contain the same number of properties.
       if (result) {
         for (key in b) {
-          if (hasOwnProperty.call(b, key) && !size--) break;
+          if (hasOwnProperty.call(b, key) && !(size--)) break;
         }
         result = !size;
       }