diff --git a/xen/arch/x86/boot/cmdline.S b/xen/arch/x86/boot/cmdline.S
index 05ffb941e5..e1f45951a9 100644
--- a/xen/arch/x86/boot/cmdline.S
+++ b/xen/arch/x86/boot/cmdline.S
@@ -138,7 +138,8 @@
         call    .Lstrlen
         add     $4,%esp
         xadd    %eax,%ebx
-        cmpb    $'\0',(%ebx)
+        /* NUL check (as $'\0' == 0x30 in GAS) */
+        cmpb    $0,(%ebx)
         je      3f
         cmpb    $' ',(%ebx)
         je      3f