Compiler edit 922: attempting to use the .ISNULL() function with a non-string argument now generates an illegal expression error.

Side note: although this addresses one mistaken use of .ISNULL(), it doesn't address the case where the argument is a string or structure variable but is less than 6 characters long. For example, this works:

IF .ISNULL($MAP(A$)) THEN ...  ! good

but this only works if VAR is an S or X type (or structure) at least 6 bytes long:

VAR = $MAP(A$)

IF .ISNULL(VAR) THEN ...     ! only good if sizeof(VAR)>=6