Index outof range error in c#

protected void lnkDownload_Click(object sender, EventArgs e)
    {

        LinkButton lnkbtn = sender as LinkButton;
        GridViewRow gvrow = lnkbtn.NamingContainer as GridViewRow;
        int ImageId = Convert.ToInt32(GridView1.DataKeys[gvrow.RowIndex].Value.ToString());
        string name, type;
        using (SqlConnection con = new SqlConnection(strCon))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandText = "select ImageName, FileName, AddedOn from ImageInfo where ImageId=@ImageId";
                cmd.Parameters.AddWithValue("@ImageId", ImageId);
                cmd.Connection = con;
                con.Open();
                SqlDataReader dr = cmd.ExecuteReader();
                if (dr.Read())
                {
                    Response.ContentType = dr["FileType"].ToString();
                    Response.AddHeader("Content-Disposition", "attachment;filename=\"" + dr["ImageName"] + "\"");
                    Response.BinaryWrite((byte[])dr["FileName"]);
                    Response.End();
                }
            }
        }
    }

Server Error in '/WebSite5' Application.

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Source Error: 


Line 149:        LinkButton lnkbtn = sender as LinkButton;
Line 150:        GridViewRow gvrow = lnkbtn.NamingContainer as GridViewRow;
Line 151:        int ImageId = Convert.ToInt32(GridView1.DataKeys[gvrow.RowIndex].Value.ToString());
Line 152:        string name, type;
Line 153:        using (SqlConnection con = new SqlConnection(strCon))

Source File: c:\Users\SURYA-BOSS\Documents\Visual Studio 2010\WebSites\WebSite5\Default.aspx.cs    Line: 151 

Stack Trace: 


[ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index]
   System.Collections.ArrayList.get_Item(Int32 index) +9368776
   System.Web.UI.WebControls.DataKeyArray.get_Item(Int32 index) +42
   _Default.lnkDownload_Click(Object sender, EventArgs e) in c:\Users\SURYA-BOSS\Documents\Visual Studio 2010\WebSites\WebSite5\Default.aspx.cs:151
   System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) +141
   System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) +150
   System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +38
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +37
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +289
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4225

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.272