diff --git a/reactos/cmake/msvc.cmake b/reactos/cmake/msvc.cmake
index c05850f43ea..d0ca86e3ef4 100644
--- a/reactos/cmake/msvc.cmake
+++ b/reactos/cmake/msvc.cmake
@@ -291,7 +291,7 @@ function(generate_import_lib _libname _dllname _spec_file)
 endfunction()
 
 if(ARCH STREQUAL "amd64")
-    add_definitions(/D__x86_64)
+    add_definitions(/D__x86_64__)
     set(SPEC2DEF_ARCH x86_64)
 elseif(ARCH STREQUAL "arm")
     add_definitions(/D__arm__)