Show message when no checkbox is checked in Bootstrap style DropDownList in ASP.Net

Hi! I used below code:

<asp:DrpDownList ID="dropFruits" runat="server" multiple="multiple">
    <asp:ListItem Text="Mango" Value="1" />
    <asp:ListItem Text="Apple" Value="2" />
    <asp:ListItem Text="Banana" Value="3" />
    <asp:ListItem Text="Guava" Value="4" />
    <asp:ListItem Text="Orange" Value="5" />
</asp:DropDownList>
<asp:Button Text="Submit" runat="server" OnClick="Save" />
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link href="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css"
    rel="stylesheet" type="text/css" />
<script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
<link href="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/css/bootstrap-multiselect.css" rel="stylesheet" type="text/css" />
<script src="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/js/bootstrap-multiselect.js" type="text/javascript"></script>
<script type="text/javascript">
    $(function () {
        $('[id*=dropFruits]').multiselect({
            includeSelectAllOption: true
        });
    });
</script>

protected void Save(object sender, EventArgs e)
{
    string selected = "";
    int selectedCount = 1;
    string selectedValues = Request.Form[dropFruits.UniqueID];
    foreach (string item in selectedValues.Split(‘,’))
    {
         selected = string.Join(",",selectedValues);
         selectedCount++;
    }
}

But I have problem. When I no select  any fruit  and click button then show message “You must select fruit” not show error. How I can solve this?