sql update

I am trying to create an update query where the tw.StrCommon field is updated to the result of the IIF but It says it cannot bind the tw.StrCommon.

The query as a select works fine.

CREATE PROCEDURE [dbo].[usp_UpdateStrCommon]

AS
BEGIN
UPDATE dbo.TblWords
SET tw.StrCommon= IIF(tsdl.StrShort IS NULL, tw.StrShort, tsdl.StrCommon)
FROM TblWords tw LEFT JOIN TblShortDescLink tsdl ON tsdl.StrShort = tw.StrShort


END

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the select query which works fine

SELECT tw.strshort,  IIF(tsdl.StrShort IS NULL, tw.StrShort, tsdl.StrCommon) AS StrTemp
FROM TblWords tw LEFT JOIN TblShortDescLink tsdl ON tsdl.StrShort = tw.StrShort

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