diff --git a/arch/um/Makefile-x86_64 b/arch/um/Makefile-x86_64
index dffd118..9558a7c 100644
--- a/arch/um/Makefile-x86_64
+++ b/arch/um/Makefile-x86_64
@@ -11,6 +11,7 @@
 CHECKFLAGS  += -m64
 AFLAGS += -m64
 LDFLAGS += -m elf_x86_64
+CPPFLAGS += -m64
 
 ELF_ARCH := i386:x86-64
 ELF_FORMAT := elf64-x86-64