Error importing CRM 2011 database into CRM 2013 organization

I am trying to import a CRM 2011 db into a CRM 2013 org (we have CRM 2013 SP1) and am getting this error.

Error| System.Exception: Error.ActionFailed Microsoft.Crm.Tools.Admin.PopulateDependenciesAction ---> Microsoft.Crm.CrmArgumentException: Expected non-empty string. ---> System.ArgumentException: Expected non-empty string.

Parameter name: entityName

Can anyone tell me how to find out WHERE this empty string is please?