SRM7 WF Fall Back Agents

I would like to configure the fall back agents for the 'Manager of Manager' shopping cart approval process to be the originating users manager. Not all departments in our organization have multiple levels of management but we would still like a second level of approval when the shopping cart amount is greater than 50K. It works when there are multiple managers defined in the org structure, but I get an Agent Determination error when there is only one manager defined.

Is there a quick way to do this?

Thanks

Jon