query expression to search by id of lookup field

How can |I construct a query in QueryExpression which will search by the id of a lookup field?

I have a scenario in which I have an incident with a known ID. I have a custom entity that is linked to the incident. I want to fetch all the related custom entities;

I have tried variations on 

            EntityReference incident = new EntityReference("incident", caseid);

            try
            {
                utilities.log("List case histories");

                QueryExpression CasesQuery = new QueryExpression
                {
                    EntityName = "qubic_servicemodule_casehistory",
                    ColumnSet = new ColumnSet(true),
                    Criteria = new FilterExpression
                    {
                        Conditions = 
                        {
                            new ConditionExpression 
                            {

                                AttributeName = "qubic_servicemodule_case",
                                Operator = ConditionOperator.Equal,
                                Values = { incident }
                            }
                        }
                    }
                };

and 

            try
            {
                utilities.log("List case histories");

                QueryExpression CasesQuery = new QueryExpression
                {
                    EntityName = "qubic_servicemodule_casehistory",
                    ColumnSet = new ColumnSet(true),
                    Criteria = new FilterExpression
                    {
                        Conditions = 
                        {
                            new ConditionExpression 
                            {
                                AttributeName = "qubic_servicemodule_caseid",
                                Operator = ConditionOperator.Equal,
                                Values = { caseid }
                            }
                        }
                    }
                };

but with no success. Fetching all the attributes for case histories shows me that I have an attribute "qubic_servicemodule_case", which in turn has an attribute which is the id for which I am searching. But how do I write a query to test that? qubic_servicemodule_case, qubic_servicemodule_case.Id and qubic_servicemodule_caseid all fail. Comparing to an entity itself fails so what should I be trying here?