Column guid is mandatory and considered unique: only one item per guid value will be created.

I am trying to do something very simple: create new nodes using Feeds.

--source (guid) is mapped to target (GUID) and is checked as 'Unique Target'.
--Update existing nodes: Do not update existing nodes (I have tried all 3 settings here too)

I create a 3 record test file with an additional header row, upload it and receive the following error no matter what I try:

Column guid is mandatory and considered unique: only one item per guid value will be created.

The guid field is in the file with values for that column being
guid
-----
1001
1002
1003

So each record is unique there.
My other data is mapped correctly.

My data clearly does not correspond to the exception being presented to me.
Is this a bug? Can someone help?