Edited Unassigned: ValidatorCalloutExtender [28015]

I'm in visual studio 2013 and the validatorcalloutextender is not firing. The other ajax items seem to be working such as the calendar extender.

Example:

<asp:DropDownList ID="ddlEmployee" runat="server" TabIndex="1" ToolTip="Employee who completed note." AppendDataBoundItems="true" DataSourceID="SqlDataSource1" DataTextField="Employee" DataValueField="Record_ID">
<asp:ListItem Selected="True" Text="--Select One--" Value="0" />
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:cn %>" SelectCommand="stpEmployee_Select_All" SelectCommandType="StoredProcedure"></asp:SqlDataSource>
<asp:RequiredFieldValidator ID="rfvEmployee" runat="server" ControlToValidate="ddlEmployee" ErrorMessage="Please select an employee who completed the note!" Font-Bold="True" ForeColor="#CC0000" InitialValue="0">*</asp:RequiredFieldValidator>
<asp:ValidatorCalloutExtender ID="vceEmployee" runat="server" TargetControlID="rfvEmployee" />

I have the latest version of Ajax 15.1.4