diff --git a/xen/common/vmap.c b/xen/common/vmap.c
index 4fd6b3067e..330e2ba897 100644
--- a/xen/common/vmap.c
+++ b/xen/common/vmap.c
@@ -53,7 +53,7 @@ static void *vm_alloc(unsigned int nr, unsigned int align,
     if ( !align )
         align = 1;
     else if ( align & (align - 1) )
-        align &= -align;
+        align = ISOLATE_LSB(align);
 
     ASSERT((t >= VMAP_DEFAULT) && (t < VMAP_REGION_NR));
     if ( !vm_base[t] )