zz = .NULL.
? ISNULL(zz) ---> .T.

? STR(zz,5) ---> Error:  Function argument value, type, or count is invalid
? STR(.NULL.,5) ---> .NULL.  

? zz+"AA" ---> Operator/Operand type mismatch
? .NULL. + "AA" ---> .NULL.

? NVL(zz+"AA","xxxxxx") ---> Operator/Operand type mismatch
? NVL(zz,"xxxxxx") ---> "xxxxxx"

USE myTable
REPLACE c1 WITH .NULL.

? c1 + "AA" ---> .NULL.
SCATTER MEMVAR
? m.c1 + "AA" ---> .NULL.

zz=c1
? zz + "AA" ---> .NULL.

zz = .NULL.
? zz + "AA" ---> .NULL.

RELEASE zz
zz = .NULL.
and all the errors return

Why do variables directly assigned .NULL. not seem to work correctly while fields and SCATTERed variables give the expected results?

Thanks........Rich