anthem:ImageButton使用时出现了问题

今天在使用anthem:ImageButton时，发现了一个问题，产生问题的过程如下
1、产生如下代码，主要是为了测试ImageButton按钮

 1<script runat="server">
 2
 3    void slowButton_Click(object sender, ImageClickEventArgs e)
 4    {
 5        System.Threading.Thread.Sleep(4000);
 6        slowResult.Text = DateTime.Now.ToString();
 7        slowResult.UpdateAfterCallBack = true;
 8    }
 9
10    void fastButton_Click(object sender, EventArgs e)
11    {
12        fastResult.Text ="good";
13        fastResult.UpdateAfterCallBack = true;
14    }
15
16</script>

C#代码

 1<form id="form1" runat="server">
 2
 3            <p>
 4                单击"Show"按钮将在服务器端引发4秒延时。如果你单击"Show"后再单击"Fast"按钮，你将先看到"Fast"的结果，再看到"Show"的结果</p>
 5            <asp:Label id="slowLabel" runat="server"
 6              AssociatedControlID="slowButton"
 7              Text="Slow:">
 8            </asp:Label>
 9            <anthem:ImageButton ID="slowButton" runat="server"
10              AlternateText="Slow"
11              ImageUrl="button_run.gif" ImageUrlDuringCallBack="button_run-down.gif"
12              OnClick="slowButton_Click">
13            </anthem:ImageButton>
14            <anthem:Label ID="slowResult" runat="server" />
15            
16            
17            <br />
18            
19            
20            <asp:Label id="fastLabel" runat="server"
21              AssociatedControlID="fastButton"
22              Text="Fast:">
23            </asp:Label>
24            <anthem:Button ID="fastButton" runat="server"
25                 Text ="click"
26              OnClick="fastButton_Click">
27            </anthem:Button>
28            
29            <anthem:Label ID="fastResult" runat="server" />
30        </form>

结果单击slowbutton后，应该返回时间的字符串，结果返回一个错误页面


有没有人能对这个问题进行回答？