diff --git a/xen/common/lib.c b/xen/common/lib.c
index 62330205fe..8ebec811b3 100644
--- a/xen/common/lib.c
+++ b/xen/common/lib.c
@@ -476,6 +476,10 @@ unsigned long long parse_size_and_unit(const char *s, const char **ps)
     case 'B': case 'b':
         s1++;
         break;
+    case '%':
+        if ( ps )
+            break;
+        /* fallthrough */
     default:
         ret <<= 10; /* default to kB */
         break;