Code not working

Hi

There are 2 records of Customer . In one first 2 character is 06. In second it is null.

IT should give error message since value is Null not 06.

IF @object_type = '2' and @transaction_type in ('A','U')
BEGIN
	SELECT TOP 1 @error = '1', @error_message = ''
	FROM CRD1 
	WHERE CRD1.CardCode = @list_of_cols_val_tab_del 
	AND (Substring(CRD1.[GSTRegnNo],1,2) <> '06' )
END

Thanks