diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 9141912ae5..87529db7d1 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -162,7 +162,7 @@ static uint32_t base_disallow_mask;
 #define L4_DISALLOW_MASK (base_disallow_mask)
 
 #define l1_disallow_mask(d)                                     \
-    ((d != dom_io) &&                                           \
+    (((d) != dom_io) &&                                         \
      (rangeset_is_empty((d)->iomem_caps) &&                     \
       rangeset_is_empty((d)->arch.ioport_caps) &&               \
       !has_arch_pdevs(d) &&                                     \