TIB_Query ParamByName() mystery, can't figure out

Dataset's atributtes:
SQL :select * from Tu(:TuID )
InsertSQL: execute procedure InsTu(:Tu,:SoID )
DeleteSQL: execute procedure DelTu(:TuID,:SoID )
CalculatedFields: TuID decimal(15,0)
KeyLinks : SoID
MasterParamLinks: TuID=TuID

CREATE PROCEDURE TU(TUID DECIMAL(15, 0))
RETURNS(SOID DECIMAL(15, 0),Name VARCHAR(33))

Place, where got error (TIB_StatementError: fieldname SoID not found):
qry->ParamByName("SoID")->Assign(qry2->FieldByName("SoID")); <- error
qry->Delete();

Tried to change qry->ParamByName("SoID") to FieldByName("SoID") error:
Tu.SoID is readonly - this is OK as I guessed

Don't understand why this happens - why can't use ParamByName? I
thought, that ParamByName are for the SP params.


Using IBO 4.2Ie, F1.5.1