I got the same error when using the method trg_validate with the customized button 'signal_draft_depheadapr' :

wf_service.trg_validate(uid, 'purchase.order', id, 'signal_draft_depheadapr', cr)

After trace I discovered that the 'id' parameter is an object one and not an integer id, I replace the above line with the following line and it worked:

wf_service.trg_validate(uid, 'purchase.order', po_obj.id, 'signal_draft_depheadapr', cr) 

The exact line that caused this problem was the cr.execute method

So check your code, the reason of this was a wrong parameter type you passed.