diff --git a/package.json b/package.json
index 8adc9d1..99469bb 100644
--- a/package.json
+++ b/package.json
@@ -21,7 +21,7 @@
     "phantomjs-prebuilt": "2.1.x"
   },
   "scripts": {
-    "postinstall": "if [ ! -d .tox ] || [ ! -d .tox/karma ]; then tox -ekarma --notest; pip install -U -t ./.tox/karma/lib/python3.5/site-packages/ ../horizon; fi",
+    "postinstall": "if [ ! -d .tox ] || [ ! -d .tox/karma ]; then tox -ekarma --notest; python3.5 -m pip install -U -t ./.tox/karma/lib/python3.5/site-packages/ ../horizon; fi",
     "test": "karma start zun_ui/karma.conf.js --single-run",
     "lint": "eslint --no-color zun_ui/static",
     "lintq": "eslint --quiet zun_ui/static"