update statement

["Jszarlik" <jszarlik@xxxxxx>]

Hello,
I would like to update one field in ATX file and I try to use SQL.
The SQL statement is following:

UPDATE ITSKCTST83/ATX SET ATX.TXIDTE
= (SELECT APH.APIRDT FROM ITSKCTST83/APH
WHERE ATX.TXVNDR=APH.APVNDR AND ATX.TXINV = APH.APINV
AND APH.PHDCYR = ATX.TXDCYR AND ATX.TXDCPX = APH.PHDCPX
AND ATX.TXDCSQ = APH.PHDCSQ AND ATX.TXCOMP = APH.APCMPY)

but I receive message:

Null values not allowed in column or variable TXIDTE

could anybody explain it and how to resolve it?

Thanks in advance
Jarek