Why GetData doesn't work with TBLOBField???

I'm trying to read and write data to TBlobFiled at run-time. Only GetDAta
according to help fills my needs. But it doesn't work - i get sth like
BDE exception. What am i doing wrong????

( tf is TBlobField , pc - PChar (allocated)
  tf.GetData(pc); {exception!!!}

Any coments welcome