Email Error

I am trying to trigger an e-mail from a another application and
i have this code which is executed - but on execution
of the code I am getting the following error ....


[Microsoft][ODBC SQL Server Driver][SQL Server]Invalid length parameter passed to the substring function.

This is the code :
SET NOCOUNT ON

DECLARE @pos INT

SELECT @pos = CHARINDEX(',','{payment_emailaddress}')

SET NOCOUNT OFF

IF @pos > 1
BEGIN
SELECT SUBSTRING('{payment_emailaddress}', 1, CHARINDEX(',','{payment_emailaddress}')-1) AS email
END
ELSE
BEGIN
SELECT '{payment_emailaddress}' AS email
END


Many thanks if anyone can help me out .....