diff --git a/gdc_client/upload/client.py b/gdc_client/upload/client.py
index 2e453622..0e4c3052 100644
--- a/gdc_client/upload/client.py
+++ b/gdc_client/upload/client.py
@@ -28,7 +28,7 @@
 
 OS_WINDOWS = platform.system() == "Windows"
 
-if not OS_WINDOWS or not portability.OS_OSX:
+if not (OS_WINDOWS or portability.OS_OSX):
     from mmap import PROT_READ
 else:
     from multiprocessing import freeze_support