diff --git a/lib/codemirror.js b/lib/codemirror.js
index <HASH>..<HASH> 100644
--- a/lib/codemirror.js
+++ b/lib/codemirror.js
@@ -966,6 +966,7 @@ var CodeMirror = (function() {
       for (var i = 0, l = lines.length; i < l; ++i)
         lines[i].stateAfter = null;
       work = [0];
+      startWorker();
     }
     function gutterChanged() {
       var visible = options.gutter || options.lineNumbers;
Make sure changing the mode actually causes a re-highlight