Entity Framework Exception: Ambiguous match found

Entity Framework Exception: Ambiguous match found

I get the error:
Ambiguous match found
During this code on the .Add
var db = new NexusEntities();
db.Reports.Add(mmr);
From googling it appears to be an issue if the there are two classes with
the same name but in different namespaces. As far as I can tell this is
not the case... Is there another reason why this error can happen? Or is
there a way to tell which parameter it is finding two of?