I have two combos. one combo lists insurance companies, the other one lists insurance policies. When the user selects an insurance company, the policies combo store should load policies given by that insurer. I have a select listener for insurer combo as shown. the parameter insurerName is the insurer whose policies we want to load.

Code:
listeners : {
                    select : {
                        fn : function(combo, value) {
                            var insurerName = combo.getRawValue();
                            Ext.Ajax.request({
                                url : './insurerController/listPolicies.json',
                                params : {
                                    insurer : insurerName
                                },
                                success : function(response) {
                                    Ext.getCmp('combo-policy').getStore().load();
                                }
                            })
                        }
                    }

                }
The policy combo is defined as shown:
Code:
{
                xtype : 'combo',
                id : 'combo-policy',
                fieldLabel : 'Policy',
                allowBlank : false,
                width : 250,
                emptyText : 'Select the Policy...',
                store : policyStore,
                displayField : 'policyName',
                valueField : 'idPolicy',
                typeAhead : true,
                triggerAction : 'all',
                editable : false,
                mode : 'local'
            }
the store for policy combo is shown below:

Code:
var policyStore = new Ext.data.JsonStore({
            proxy : new Ext.data.HttpProxy({
                url : './insurerController/listPolicies.json'
            }),
            totalProperty : 'count',
            remoteSort : false,
            autoload : true,
            root : 'result',
            fields : ['idPolicy', 'policyName', 'policyLimit', 'policyType', 'inpatientLimit', 'outPatientLimit', 'premium', 'idInsurer']
        });
The problem is that when I select an insurer from the insurer combo, I get 2 http requests the first of which gives the correct result but the second throws an exception saying that insuerName cannot be null.

what is the cause of the problem and how can it be solved.