Reseting p:ajax drop down after un-selecting option - jsf

Question

I am using prime faces in my project. I am generating a pull down by using p:ajax and its working fine for me. But i have notice a problem that is if i select ---select one-- my pull down which i have generated by p:ajax is not resetting to default .
My code is:
<p:selectOneMenu id="propExam" value="#{rollNoBean.examination}"
converter="omnifaces.SelectItemsConverter" editable="false"
required="true" label="Examination Applied For" styleClass="dropdownWidth">
<f:selectItem itemLabel="---Select One---" itemValue="0" />
<f:selectItems value="#{rollNoBean.testExamNames}" var="test" itemLabel="#{test.name}" itemValue="#{test}" noSelectionOption="true" />
<p:ajax listener="#{rollNoBean.readFilterByExamination(rollNoBean.examination.id)}"
update="degreeList centerId" />
</p:selectOneMenu>
<h:outputText value="Select Filter Category :" />
<p:selectOneMenu id="degreeList"
value="#{rollNoBean.filter}" editable="false"
converter="omnifaces.SelectItemsConverter" required="true"
label="Please Select degree" styleClass="dropdownWidth">
<f:selectItem itemValue="" itemLabel="---Select One---" />
<f:selectItems value="#{rollNoBean.degreeNames}"
var="degree" itemLabel="#{degree.name}" itemValue="#{degree}" />
</p:selectOneMenu>
and my bean code is:
public List<Filter> readFilterByExamination(int id)
{
if(id!=0)
{
return degreeNames=examinationDetailsService.readFilterByExamination(id);
}
return degreeNames=new ArrayList<Filter>();
}
Please give suggetion