Help with an IIF statement please

Hello

The following IIF expression isn't working for me:

IIF(Fields!Number.Value BETWEEN 1 AND 1000, Fields!Number.Value, "all ok")

Basically I want to list the value if it falls within 1 and 100 and show 'all ok' if it doesn't.

Please can you advise where I've gone wrong as this is throwing an error.

Thanks

WF