java.lang.RuntimeException: Invalid action class configuration that references an unknown class named [userRegisterAction]

<s:form action="UserRegisterAction">
<s:textfield name="custorm.name" label="姓名" required="true"/>
<s:password name="custorm.password" label="密码" required="true"/>
<s:submit value="注册"/>
<s:reset value="重填"/>
</s:form>

private Custorm custorm;
private ServiceDao serviceDao;
public Custorm getCustorm() {
return custorm;
}
public void setCustorm(Custorm custorm) {
this.custorm = custorm;
}
public ServiceDao getServiceDao() {
return serviceDao;
}
public void setServiceDao(ServiceDao serviceDao) {
this.serviceDao = serviceDao;
}

public String execute() throws Exception{
this.serviceDao.saveCustorm(this.custorm);
return SUCCESS;

}

<validators>
<field name="custorm">
<field-validator type="visitor">
<param name="context">user</param>
<param name="appendPrefix">true</param>
<message>用户的</message>
</field-validator>
</field>
</validators>