Opportunity workflow error - Rep%

I have a workflow step that is updating the Sales Stage.  The Sales Stage is not a read only field and the field is not available to have field validation rules on it.  The workflow in question is a When Modified Record Saved and is 1st to execute (order #).  On the step that updates the Sales Stage I get an error as follows:

" This operation is not available for read only field 'Rep %'.(SBL-DAT-00402) ".  

I've seen this before but can't figure out if this represents a real field or a generic message.  The sales stage itself is not read only nor are any of the other fields being changed in the other workflow steps.  I am not running any java script.