I have run into a challenge that has stopped me in my tracks. Hopefully someone else has some input as to what I am missing or doing wrong because I have spent the last several days making futile attempts to figure it out. I have a C# utility that gathers information from an external source and uses web service calls to create new opportunities in NetSuite. The following error is returned from the call to the AddOpportunity function:

The specified custom field reference CUSTOMLIST_OPP_TYPE is invalid.

Here is the code where I set the property for the custom field. Outside of this snippet I create an Opportunity object referred to here as "opportunity".