diff --git a/lib/System/Atomic.cpp b/lib/System/Atomic.cpp
index 8c61ff1069f..cefd0bbf553 100644
--- a/lib/System/Atomic.cpp
+++ b/lib/System/Atomic.cpp
@@ -18,6 +18,7 @@ using namespace llvm;
 
 #if defined(_MSC_VER)
 #include <windows.h>
+#undef MemoryFence
 #endif
 
 void sys::MemoryFence() {