How to Set Value to PartyList

Hello Guys,

I'm struggling to set value to partylist in campaign response entity. Currently we are working in integration project

Here is the code

AJP.Xrm.JobWorkers.Proxy.activityparty[] _customer = new AJP.Xrm.JobWorkers.Proxy.activityparty[1];
_customer[0] = new AJP.Xrm.JobWorkers.Proxy.activityparty();
_customer[0].partyid = new Lookup(EntityName.contact.ToString(), jobcoders.jl_customerid.Value);
_newCampaignsResponse.customer = _customer;

It throws me error "The given key was not found in the dictionary".

But I checked in the DB the customer is there. Please help me to solve this problem