diff --git a/requirements.txt b/requirements.txt
index 0dcbc92..0ffe1f1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
-mock==2.0.0; python_version < '3.0'
+mock==4.0.0; python_version < '3.0'
 pyobjc-core==4.1; sys_platform == 'darwin'
 pytest==3.5.0
 pytest-cov==2.5.1