Error: -1

[zarsoft]

DIM s AS STRING
s = ""
PRINT LEN s-1 ' gives 65535
PRINT LEN ""-1 ' gives -1

[boriel]

That's a casting problem (inconsistency).
LEN (string) is of type UInteger, so 65535 makes sense.
The second statement, LEN "" is being evaluated to 0 in compile time, hence converted to Integer (Do you get a Warning with these?)

I will have a look into it, however.

If you want to ensure you get -1, use Cast(Integer, LEN s) to ensure it's always Integer (-32768 to 32767).