search for all messages related to a specific entity

I need to load all the message items for a particular customer.  I searched the forums and found a couple different posts that gave some info:
Search for Messages...
Access a list of contacts...

The problem I'm having is with the filter that joins a message to a customer.  Here is the code I'm trying to use:

 var filters = new Array();  filters[0] = new nlobjSearchFilter( 'messagetype', null, 'is', 'FAX', null);  filters[1] = new nlobjSearchFilter( 'internalid', 'customer',       'equalto', patient.getFieldValue('internalid'), null);    var searchresults = nlapiSearchRecord( 'message', null, filters, columns); 

When I run the above code I get the following error:

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