Problems with CollapsiblePanelExtender

I'm using VS 2012, framework 4.5.  I want to add a CollapsiblePanelExtender and display "+" and "-" to expand and collapse.  I can't seem to make this work.  My panel does expand and collapse but only the "+" displays.  How can I make this toggle back and forth?  Here is my code:

<asp:Panel ID="Panel1" runat="server" CssClass="pnl" Width="40%">
<div style="float:left;">
</div>
<div style="float:right;">
<asp:Label ID="Label1" runat="server" Text="+"></asp:Label>
</div>
<div style="clear:both"></div>
</asp:Panel>
<asp:Panel ID="Panel2" runat="server" Height="0" Style="border: solid 1px #c0c0c0; padding: 5px;" Width="40%">
<h3>Panel2</h3>
<p>
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
</p>
</asp:Panel>
<cc1:CollapsiblePanelExtender
ID="CollapsiblePanelExtender1"
runat="server"
CollapseControlID="Panel1"
Collapsed="true"
ExpandControlID="Panel1"
ExpandDirection="Vertical"
TargetControlID="Panel2"
ScrollContents="false"
CollapsedText="+"
ExpandedText="-">
</cc1:CollapsiblePanelExtender>