Digging deeper, I see the following in the inner exception:
Message=Cannot create converter for SQL type: 'uniqueidentifier' CLR type: ''.
I have identified the error as being a relationship where the foreign key is a GUID, but the GUID is not the primary key in the source entity. When I use the Mapping Details Editor, the only choices I have for the source are the primary key and "<None>".
Is it not possible to use a non-primary key value as a foreign key? This seems to me to be a fairly basic requirement.