Tds @ higher rates u/s 206aa

Our assessee has file a TDS regular statement with quoting "PANNOTAVBL" for an assessee. It was due to mistake of the staff and the valid PAN was available to the assessee. Hence, a correction satatement has been filed with correcting the mistake and valid PAN of the assessee has been quoted. Later TDS CPC issue demand notice stating "INVALID PAN" and required to do TDS @ 20% (sec. 206AA) Because of PAN ERROR.

Expert are requested to give opinion on that matter and what should we have to do ?