diff --git a/setup.py b/setup.py
index f8c3ff2..1f4a6a0 100644
--- a/setup.py
+++ b/setup.py
@@ -23,7 +23,7 @@ except ImportError:
 
 tests_require = ['pytest']
 if mock is None:
-    tests_require += ['mock']
+    tests_require.append('mock')
 
 
 requires = [