diff --git a/Modules/CMakeDetermineSystem.cmake b/Modules/CMakeDetermineSystem.cmake
index 20c1541ba..3a95d2a6f 100644
--- a/Modules/CMakeDetermineSystem.cmake
+++ b/Modules/CMakeDetermineSystem.cmake
@@ -73,7 +73,7 @@ if(CMAKE_HOST_UNIX)
 else()
   if(CMAKE_HOST_WIN32)
     set (CMAKE_HOST_SYSTEM_NAME "Windows")
-    if (ENV{PROCESSOR_ARCHITEW6432})
+    if (DEFINED ENV{PROCESSOR_ARCHITEW6432})
       set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITEW6432}")
     else()
       set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITECTURE}")