Non integer Page number in paging mold="os"

Press next button in paging several times,
The page number will be non integer.

<zk>
<window>
<zscript><![CDATA[
List items = new java.util.AbstractList() {
public int size() {
return 5000;
}

public Object get(int j) {
return new Integer(j);
}
};

]]>
</zscript>
<paging id="pg" mold="os" totalSize="100" pageIncrement="3">
</paging>
<grid width="300px" mold="paging" paginal="${pg}">
<columns>
<column label="Left" />
<column label="Right" />
</columns>
<auxhead>
<auxheader label="Whole" colspan="2" />
</auxhead>
<rows>
<row forEach="${items}">
<label value="${each}" />
<label value="${each}" />
</row>
</rows>
</grid>
</window>
</zk>