Cannot filter message search by internal ID?

Hi-

We have an after submit user event script that creates a task and marks it as complete every time that an outgoing email is sent to a customer or contact. The script only catches outgoing emails, so step one is to figure out if the message is outgoing or incoming. To do this we need to grab the messages internal ID, and run a search with the column 'isincoming'. The problem is that  we keep getting the following error:

"An nlobjSearchFilter contains an invalid operator, or is not in proper syntax: internalid."

I have tried both--

var messageid = nlapiGetRecordId(); //Get Message internal ID