diff --git a/core/rfb.js b/core/rfb.js
index f40c7d78..b60a1198 100644
--- a/core/rfb.js
+++ b/core/rfb.js
@@ -669,7 +669,7 @@ RFB.prototype = {
         if (down) {
             this._mouse_buttonMask |= bmask;
         } else {
-            this._mouse_buttonMask ^= bmask;
+            this._mouse_buttonMask &= ~bmask;
         }
 
         if (this._viewportDrag) {