diff --git a/lib/helper.js b/lib/helper.js
index e93acb181..4bc7ee46c 100644
--- a/lib/helper.js
+++ b/lib/helper.js
@@ -131,7 +131,7 @@ exports.formatTimeInterval = function (time) {
 }
 
 var replaceWinPath = function (path) {
-  return exports.isDefined(path) ? path.replace(/\\/g, '/') : path
+  return _.isString(path) ? path.replace(/\\/g, '/') : path
 }
 
 exports.normalizeWinPath = process.platform === 'win32' ? replaceWinPath : _.identity