diff --git a/packages/windows/installer/NTMakefile b/packages/windows/installer/NTMakefile
index 54af6064b..77a2491fc 100644
--- a/packages/windows/installer/NTMakefile
+++ b/packages/windows/installer/NTMakefile
@@ -110,10 +110,12 @@ CRTNAME=CRT
 CRTNAME=DebugCRT
 !endif
 
+!if "$(MMDIR)"==""
 MMDIR=$(ProgramFiles)\Common Files\Merge Modules
 !if !exist($(MMDIR))
 MMDIR=$(SystemDrive)\Program Files (x86)\Common Files\Merge Modules
 !endif
+!endif
 
 !if exist("$(MMDIR)")