Adding "AND fixversion not in ("USX Backlog")" excludes EMPTY versions also

If I add "AND fixVersion not in ("USX Backlog")" to my query not only are "USX Backlog" versions excluded, but also any issues with a EMPTY version.

Project in ("Customer Issues", "TIS Suite") AND status not in (Verification, Done, Closed) AND fixVersion not in ("USX Backlog") AND assignee in (membersOf(Documentation)) ORDER BY key DESC, "Customer Temperature" DESC, priority DESC

I am using;

Atlassian JIRA Project Management Software (v6.3.14#6345-sha1:47b2bb0)