paper-dropdown-menu doesnt show errorMessage on validation

[giulian...@gmail.com]

Hi, i've some problem with paper-dropdown-menu:

When i made the validation of the selection, it works perfectly and the framework show the Red underline for the error but it doesnt show the Error-Message.

Anyone can help me?

This is the code:

<paper-dropdown-menu label="[[label]]" class="fullwidth" error-message="[[errorMessage]]" required="[[required]]" id="[[id]]_ddMenu" placeholder="[[placeholder]]">
    <paper-menu class="dropdown-content" attr-for-selected="valore" selected="{{selectedItem}}">
       <paper-item valore="val1">Label 1</paper-item>
       <paper-item valore="val2">Label 2</paper-item>
       <paper-item valore="val3">Label 3</paper-item>
    </paper-menu>
</paper-dropdown-menu>