diff --git a/lib/ace/mode/text_highlight_rules.js b/lib/ace/mode/text_highlight_rules.js
index fc7bfb67..48e016b0 100644
--- a/lib/ace/mode/text_highlight_rules.js
+++ b/lib/ace/mode/text_highlight_rules.js
@@ -212,6 +212,11 @@ var TextHighlightRules = function() {
             for (var i = list.length; i--; )
                 keywords[list[i]] = className;
         });
+        // in old versions of opera keywords["__proto__"] sets prototype
+        // even on objects with __proto__=null
+        if (Object.getPrototypeOf(keywords)) {
+            keywords.__proto__ = null;
+        }
         this.$keywordList = Object.keys(keywords);
         map = null;
         return ignoreCase