diff --git a/tools/libxl/libxl_json.c b/tools/libxl/libxl_json.c
index 98335b05b2..346929a2b1 100644
--- a/tools/libxl/libxl_json.c
+++ b/tools/libxl/libxl_json.c
@@ -1013,7 +1013,7 @@ out:
 yajl_gen_status libxl__uint64_gen_json(yajl_gen hand, uint64_t val)
 {
     char *num;
-    unsigned int len;
+    int len;
     yajl_gen_status s;