diff --git a/lib/less/rhino.js b/lib/less/rhino.js
index 70531265..aea6731a 100644
--- a/lib/less/rhino.js
+++ b/lib/less/rhino.js
@@ -1,7 +1,7 @@
 /*jshint rhino:true, unused: false */
 /*global name:true, less, loadStyleSheet */
 
-if (initRhinoTest) { // definition of additional test functions (see rhino/test-header.js)
+if (typeof initRhinoTest === 'function') { // definition of additional test functions (see rhino/test-header.js)
     initRhinoTest();
 }