diff --git a/js/libs/jscssp/jscssp.js b/js/libs/jscssp/jscssp.js
index 01c79e1..7858fee 100644
--- a/js/libs/jscssp/jscssp.js
+++ b/js/libs/jscssp/jscssp.js
@@ -4401,7 +4401,7 @@ jscsspStylesheet.prototype = {
 
   deleteRule: function(aIndex) {
     try {
-      this.cssRules.splice(aIndex);
+      this.cssRules.splice(aIndex, 1);
     }
     catch(e) {
     }
@@ -5428,4 +5428,4 @@ function FilterRepeatingGradientForOutput(aValue, aEngine)
   return "";
 }
   return CSSParser;
-})
\ No newline at end of file
+})