CREATE TABLE [dbo].[Temp] (
    [Word] VARCHAR (MAX) NULL,
    [SFI]  REAL          NULL,
    [U]    INT           NULL,
    [D]    REAL          NULL
);

MERGE [dbo].[Word] as target
USING [dbo].[Temp] as source
ON target.[WordId] = source.[Word]
WHEN MATCHED THEN
    UPDATE SET [NAWL963SFI] = source.[SFI], [NAWL963U] = source.[U], [NAWL963D] = source.[D]
WHEN NOT MATCHED THEN
    INSERT ([WordId],[NAWL963SFI],[NAWL963U], [NAWL963D], [NAWL963])
    VALUES (source.[Word],source.[SFI],source.[U], source.[D], 1);

Msg 515, Level 16, State 2, Line 41
Cannot insert the value NULL into column 'WordId', table 'ab.dbo.Word'; column does not allow nulls. UPDATE fails.
The statement has been terminated.

SELECT count(*) from Temp where Word = null;