Update query validation errors

From: jp (anonymous_at_discussions.microsoft.com)
Date: 09/29/04 

Date: Wed, 29 Sep 2004 12:22:55 -0700

The following query occasionally gives me validation rule
violations and I suspect it may be because sometimes
the "update from" fields are empty. The CMaterial and
CRatio and CContract fields are not required and are the
same type as the "from" fields. I am verifying that
SampleId exists in tblMfg before running the query. Do I
need to handle nulls in my update and if so can someone
please show me how or is something else causing me
problems?
UPDATE tblMfg INNER JOIN tblCombineComp ON tblMfg.SampleID
= tblCombineComp.SampleID SET tblMfg.CMaterial =
[tblCombineComp].[Material], tblMfg.CContracts =
[tblCombineComp].[Contracts], tblMfg.CRatio =
[tblCombineComp].[Ratio];