Hi, I have a problem using Form and reader, when de form is load (form.load()) and server dont retrive data, i get the error:

root has no properties
/js/ext/ext-all-debug.js Line 10402

Line 10402:
Code:
var root = this.getRoot(o), c = root.length, totalRecords = c, success = true;
Is this a bug or I have wront my code???
Is there a way to fixed???

My code:

Code:
form= new Ext.form.Form({
                 autoCreate:false,
                     labelAlign: 'right',
                     labelWidth: 180,
                     reader : new Ext.data.JsonReader({
                                  id:'IDPROMEP',
                                  root: 'rows', 
                                  totalProperty: 'total',
                                  successProperty:'success'
                                             }, [ {name:'IDPROMEP'   , mapping:'IDPROMEP'},
                                                    {name:'APEPAT'     , mapping:'APEPAT'},
                                                    {name:'APEMAT'     , mapping:'APEMAT'},
                                                    {name:'NOMBRES'    , mapping:'NOMBRES'},
                                                    {name:'GENERO'     , mapping:'GENERO'},
                                                    {name:'RFC'        , mapping:'RFC'},
                                                    {name:'CURP'       , mapping:'CURP'},
                                                    {name:'CORREO'     , mapping:'CORREO'},
                                                    {name:'CORREOALT'  , mapping:'CORREOALT'},
                                                    {name:'AREADEDI'   , mapping:'AREADEDI'},
                                                    {name:'DISIPLIDEDI', mapping:'DISIPLIDEDI'},
                                                    {name:'EDOACTU'    , mapping:'EDOACTU'}
                                             ])
                        });


form.load({ url:'idptc/process.php',
                             params:{ action:'load'},
                                           method: 'POST',
                                           waitMsg:'Loading...',
                                           failure: function () { this.loadFailure();}
            });
and php response:

({"success":true, "total":"0", "rows":null})