diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index e9c4b2b38803..92ea1670cf1c 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -8,6 +8,7 @@
 
 #include <sysdep/ptrace_user.h>
 #include <generated/asm-offsets.h>
+#include <linux/stddef.h>
 
 #define STUB_MMAP_NR __NR_mmap
 #define MMAP_OFFSET(o) (o)