diff --git a/arch/um/Makefile-x86_64 b/arch/um/Makefile-x86_64
index 8836f0d..bcfd6ea 100644
--- a/arch/um/Makefile-x86_64
+++ b/arch/um/Makefile-x86_64
@@ -14,7 +14,7 @@
 CHECKFLAGS  += -m64
 KBUILD_AFLAGS += -m64
 LDFLAGS += -m elf_x86_64
-CPPFLAGS += -m64
+KBUILD_CPPFLAGS += -m64
 
 ELF_ARCH := i386:x86-64
 ELF_FORMAT := elf64-x86-64