REST Adapter - optional parameters

Hi,

I'm trying to do a GET with an empty parameter to a sap rest receiver adapter:

URL pattern is like this:

http://Host:Port/rest?tounit={tounit}&fromunit={fromunit}

Example with empty param::

http://Host:Port/rest?tounit=123&fromunit=

I get the error

Transmitting the message using connection REST_http://sap.com/xi/XI/System failed, due to: com.sap.engine.interfaces.messaging.api.exception.MessagingException: com.sap.aii.adapter.rest.ejb.receiver.PlaceholderMissingException: URL placeholder fromunit is not configured, or has an empty value

Is this standard beaviour or can I somehow tell the adapter to accept empty parameters?

Note that I put the parameter as Optional in the sender adapter.

Thanks