SWITCH Incorrect Argument Amount - Can't See Where - ms-access

Question

I'm trying to use the SWITCH function in MS Access and I keep getting told there aren't the right number of arguments. I'm new to the switch function but I understand the syntax.
Switch ( expression1, value1, expression2, value2, ... expression_n, value_n )
Am I doing something in the switch that can't be used, the sub-queries maybe? I've checked my commas, parentheses and that there is an expression and then a return value. Driving me crazy as I'm converting from T-SQL to jet and replacing a CASE statement that worked perfectly fine.
Switch(
(eh.STARTDTE <> sh.STARTDTE AND IsNull(eh.ENDDTE,"") <> IsNull(sh.ENDDTE,"") AND eh.STARTDTE < sh.STARTDTE),
"FAIL - Employment Start Date Before Service Start Date",
(eh.STARTDTE <> sh.STARTDTE AND IsNull(eh.ENDDTE,"") <> IsNull(sh.ENDDTE,"") AND eh.ENDDTE > sh.ENDDTE),
"FAIL - Employment End Date After Service End Date",
(eh.STARTDTE <> sh.STARTDTE AND IsNull(eh.ENDDTE,"") <> IsNull(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd(dd,-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND IsNull(eh.ENDDTE,"") <> IsNull(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNo
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd(dd,-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND IsNull(eh.ENDDTE,"") <> IsNull(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.STARTDTE = DateAdd(dd,1,eh.ENDDTE)
) = 0),
"FAIL - Next Employment Period Corrupt or Missing",
(eh.STARTDTE = sh.STARTDTE AND eh.ENDDTE <> sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.STARTDTE = DateAdd(dd,1,eh.ENDDTE)
) = 0),
"FAIL - Next Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND eh.ENDDTE = sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd(dd,-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE = sh.STARTDTE AND eh.ENDDTE = sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
) <> 1),
"FAIL - Too Many or No Employment History Records"
) AS "Reason"
UPDATED WITH ANSWERS
Switch(
(eh.STARTDTE <> sh.STARTDTE AND NZ(eh.ENDDTE,"") <> NZ(sh.ENDDTE,"") AND eh.STARTDTE < sh.STARTDTE),
"FAIL - Employment Start Date Before Service Start Date",
(eh.STARTDTE <> sh.STARTDTE AND NZ(eh.ENDDTE,"") <> NZ(sh.ENDDTE,"") AND eh.ENDDTE > sh.ENDDTE),
"FAIL - Employment End Date After Service End Date",
(eh.STARTDTE <> sh.STARTDTE AND NZ(eh.ENDDTE,"") <> NZ(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd("dd",-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND NZ(eh.ENDDTE,"") <> NZ(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNo
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd("dd",-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND NZ(eh.ENDDTE,"") <> NZ(sh.ENDDTE,"") AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.STARTDTE = DateAdd("dd",1,eh.ENDDTE)
) = 0),
"FAIL - Next Employment Period Corrupt or Missing",
(eh.STARTDTE = sh.STARTDTE AND eh.ENDDTE <> sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.STARTDTE = DateAdd("dd",1,eh.ENDDTE)
) = 0),
"FAIL - Next Employment Period Corrupt or Missing",
(eh.STARTDTE <> sh.STARTDTE AND eh.ENDDTE = sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
AND eh2.ENDDTE = DateAdd("dd",-1,eh.STARTDTE)
) = 0),
"FAIL - Previous Employment Period Corrupt or Missing",
(eh.STARTDTE = sh.STARTDTE AND eh.ENDDTE = sh.ENDDTE AND (
SELECT
COUNT(*)
FROM
emphist AS eh2
WHERE
eh2.MEMBNO = sh.MEMBNO
AND eh2.EMPID = sh.EMPID
) <> 1),
"FAIL - Too Many or No Employment History Records"
) AS "Reason"

Answer 1

It is not Switch but IsNull that raises the error. So replace all these faulty statements:
IsNull(eh.ENDDTE,"")
with:
Nz(eh.ENDDTE)

Answer 2

OK, there were two issues.
The IsNull function isn't an Access function so I replaced with NZ (same syntax).
For the DateAdd function, I had neglected to surround the datepart argument with speech marks.
The query now happily runs. I'm guessing as the IsNull function wasn't recognised, it appeared as though the argument was missing which I suppose it was. Lesson learnt, incorrect number of arguments could also indicate invalid functions.
NB. I will update my OP with the amended switch statement for others to compare.