为什么我在硬编码值上得到“不能将值NULL插入到'X'列”错误?

INSERT INTO DDA_MatchStrings 
(Unit, FamilyID, AccountNumber, MatchType,  MatchString, SnapshotDate,  IgnoreRecord,   Merged)
SELECT 
 Unit, FamilyID, AccountNumber, 'EMAIL',    MatchString, @SnapshotDate, 0,              0
FROM DDA_MatchStrings_temp AS tms 
WHERE tms.MatchString IN 
    (SELECT tms.MatchString FROM DDA_MatchStrings_temp AS tms
    GROUP BY tms.MatchString
    HAVING COUNT(DISTINCT unit) > 1)
ORDER BY tms.MatchString
INSERT INTO DDA_MatchStrings 
(Unit, FamilyID,