Grid cast class exception

[Szemere Szemere]

I'm getting a strange cast class exception with the grid component, along
the lines of:
Render queue error in BeginRender[admin/WidgetList:grid.rows.gridcell]:
com.companyname.data.Widget cannot be cast to
com.companyname.data.Widget_$$_javassist_0

The code is simple and clean and I don't get it elsewhere, so I'm puzzled.
Any ideas of where to look?

The code is similar to the following (names changed to protect the
innocent):

tml includes:
        <t:grid t:pagerPosition="both"
            source="widgetList"
            row="widgetLoop"
            rowsPerPage="50" >

            <t:parameter name="nameCell">
                   <t:pagelink page="admin/stuff/WidgetEdit"
context="widgetLoop.id">
                       ${widgetLoop.name}
                   </t:pagelink>
            </t:parameter>

        </t:grid>

.java:
package com.companyname.web.pages.admin;

import java.util.List;

import org.apache.tapestry5.annotations.Property;
import org.apache.tapestry5.ioc.annotations.Inject;
import org.hibernate.Session;

import com.companyname.data.Widget;
import com.companyname.web.pages.SecuredPage;

public class WidgetList extends SecuredPage {
    @Inject private Session session;

    @Property List<Widget> widgets;
    /**
     * stores current group for loop usage
     */
    @Property private Widget widgetLoop;

    void onActivate(Object[] context) {
        //no-op
    }

    @SuppressWarnings("unchecked")
    void setupRender() {
        widgets = (List<Widget>)
session.createCriteria(Widget.class).list();
    }

    public List<Widget> getWidgetList() {
        return widgets;
    }
}

package com.companyname.data;

import java.util.Date;
import java.util.GregorianCalendar;
import java.util.HashSet;
import java.util.Set;

public class Widget {
    public static final String USEFUL_NAME = "help";

    private Long id;
    private String name;
    private Set<Item> items = new HashSet<Item>();
    private Date createdDate;
    private String description;

    public Widget() {
        //if this is a persistent instance, then the createdDate will be
        //recovered from the DB and set post-constructor
        createdDate = new GregorianCalendar().getTime();
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<Item> getItems() {
        return items;
    }

    public void setItems(Set<Item> items) {
        this.items = items;
    }

    public Date getCreatedDate() {
        return createdDate;
    }

    public void setCreatedDate(Date createdDate) {
        this.createdDate = createdDate;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}