ASP image button onclick event does not fire in IE, works in Firefox and chrome

Question

Onclick event of imagebutton (inside usercontrol) does not get fired in IE, it works in Firefox and chrome. The usercontrol contains some required field validations as well as a popup.

Code below:

  <asp:ImageButton ID="btnSend" ImageUrl="~/images/send.gif"  
  ValidationGroup="SiteFeedback" CausesValidation="true" runat="server" 
  TabIndex="5" OnClick="btnSend_Click"></asp:ImageButton>


   protected void btnSend_Click(object sender, ImageClickEventArgs e)
{
    long? userID = null;
    string name = this.txtName.Text;
    string message = this.txtMessage.Text;
    int regarding = Convert.ToInt32(this.ddlRegarding.SelectedValue);
    string email = this.txtEmail.Text;
    int responseType = (int)CommonHelper.ResponseType.Feedback;
    if (Session["User_Userid"] != null) userID = 
      Convert.ToInt64(Session["User_Userid"]);
    bool isSuccess = new ManageContactUs().SaveContactUs(userID, name, message, 
    regarding, email, responseType);
    if (isSuccess) this.SendMail();   
 }