diff --git a/monitor.c b/monitor.c
index e495c50967..9a18639c4b 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1584,8 +1584,11 @@ static target_long expr_unary(void)
         n = 0;
         break;
     default:
-        /* XXX: 64 bit version */
+#if TARGET_LONG_BITS == 64
+        n = strtoull(pch, &p, 0);
+#else
         n = strtoul(pch, &p, 0);
+#endif
         if (pch == p) {
             expr_error("invalid char in expression");
         }