T5 : Getting Error when invoke Form

[Penyihir Kecil]

i always getting this error message :

Processing of request failed with uncaught exception: An event handler for
component admin/FormFormation:formationform returned the value true (from
method org.apache.tapestry.corelib.components.Form.onAction(java.lang.Object[])
(at Form.java:283)).  Return type java.lang.Boolean can not be handled.
Configured return types are java.lang.Class, java.lang.String,
org.apache.tapestry.Link, org.apache.tapestry.StreamResponse,
org.apache.tapestry.runtime.Component.


my code :
<form t:type="Form" t:id="formationForm">
      <t:errors/>
      <t:label for="formationField"/> :
      <input t:type="TextField" t:id="formationField" value="
Formation.formation" validate="required,minlength=3" size="20"
label="username"/>
       <input type="submit" value="ok"/>
 </form>


----
void onPrepare(){
        if (formation == null){
            formation = new Formation();

        }
    }

    Object onSuccessFromFormationForm(){
        return GridFormation.class;
    }
    public IFormationDao getFormationDao() {
        return formationDao;
    }

    public Formation getFormation() {
        return formation;
    }
------


could anyone could explain me why....?