I´m getting a ClassCastException when I using a empty option in a selectOneMenu. Does anybody can help me?

Thanks in advance!

java.lang.ClassCastException: java.lang.String
 at org.jboss.seam.ui.EnumConverter.getAsString(EnumConverter.java:28)
 at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.getFormattedValue(HtmlBasicRenderer.
java:469)

<h:selectOneMenu value="#{politicapublica.classificacao}">
 <f:selectItem itemValue="" itemLabel=" " />
 <s:convertEnum enumClass="myEnumClass" />
 <s:enumItem enumValue="FISCAL" label="FISCAL" />
 <s:enumItem enumValue="SEGURIDADE" label="SEGURIDADE" />
</h:selectOneMenu>