diff --git a/src/coord/axisModelCommonMixin.js b/src/coord/axisModelCommonMixin.js
index ea79d57495acb350f26fe7a3e0feec8a50ba4ab6..71d0a32465c2574a37c8453c39ea12baf8d5841a 100644
--- a/src/coord/axisModelCommonMixin.js
+++ b/src/coord/axisModelCommonMixin.js
@@ -8,7 +8,7 @@ define(function (require) {
             return obj.value;
         }
         else {
-            return obj;
+            return obj + '';
         }
     }