Customer Center throws "The author internal ID or email must match an employee" error

Both customers and prospects can log into our Customer Center, once granted access. I have a custom script which checks a particular field on the customer record.

If the visitor is a customer, no errors happen.

If the visitor is a prospect, an error is thrown while fetching their record. The script contains the following and fails at the nlapiLoadRecord:

     var ctx = nlapiGetContext();      var last_update_pref = ctx.getSetting('SCRIPT', 'custscript_eula_lastupdated');      var user_id = ctx.getUser();        nlapiLogExecution('AUDIT', 'showEULA start', 'Starting with user_id ' + user_id);        var rec = nlapiLoadRecord('customer', ctx.getUser());        // ERROR HERE, NEVER LOGS THE FOLLOWING:      nlapiLogExecution('AUDIT', 'showEULA fetch', 'Customer record fetched for customer ' + user_id);