Query expression using customerid

I need to write a query expression using customerid but the below query is throwing format exception. Please help

QueryExpression query = new QueryExpression("opportunity");
Guid contactid = new Guid("23E62DCE-F154-E843-814K-005056BB4062");
query.Criteria.AddCondition("customerid",ConditionOperator.Equal,new EntityReference("contact",contactid));
EntityCollection result = _orgService.RetrieveMultiple(query);