diff --git a/tests/mypy_test.py b/tests/mypy_test.py
index a90667e03..cb042d83f 100755
--- a/tests/mypy_test.py
+++ b/tests/mypy_test.py
@@ -131,6 +131,7 @@ def main():
             runs += 1
             flags = ['--python-version', '%d.%d' % (major, minor)]
             flags.append('--strict-optional')
+            flags.append('--no-site-packages')
             if args.no_implicit_optional:
                 flags.append('--no-implicit-optional')
             if args.warn_unused_ignores: