diff --git a/package.json b/package.json
index dddfef2..46a0cea 100644
--- a/package.json
+++ b/package.json
@@ -7,7 +7,7 @@
   "repository": "math-utils/hierarchical-clustering",
   "dependencies": {},
   "devDependencies": {
-    "mocha": "2",
+    "mocha": "2.4.1",
     "istanbul": "0"
   },
   "scripts": {