Primefaces SelectCheckboxMenu multiple label doesn't appears

I'm trying to implement Primefaces 6.1 SelectCheckboxMenu with multiple="true". But it doesn't shows the label. Even in component given in SelectCheckboxMenu Showcase, you can remark that it has an empty value for the label

UPDATE

code snippet from showCase

    <h:outputLabel for="multiple" value="Multiple:" />
    <p:selectCheckboxMenu id="multiple" 
               value="#{checkboxView.selectedCities2}" label="Cities" 
               multiple="true" filter="true" filterMatchMode="startsWith" 
               panelStyle="width:250px">
        <f:selectItems value="#{checkboxView.cities}" />
    </p:selectCheckboxMenu>

error snapshot from showcase

So, there is no solution to that problem ??