Issue Statistics Gadget fails when field value is None

I am trying to use the Issue Statistics Gadget with a custom field "Category".
The Category custom field is of type select list.
When one of the issues has a Category field value not set (Note) the gadget will fail with following error.
It only happens when I do as sort by Total.

An internal server error occurred when requesting resource http://bcmhw8070.ca.nortel.com:8080/rest/gadget/1.0/stats/generate?projectOrFilterId=filter-13140&statType=customfield_10107&includeResolvedIssues=false&sortDirection=asc&sortBy=total.