Using RequiredFieldValidator in GridView using C# in ASP.Net

I have gridview control on my web page and I allow the user to add a serial number to a textbox in each row of the gridview.  If they do not enter a serial number, I want to display a message.  Here is my ASP.Net code:

<tr>
    <td>
            <asp:GridView ID="gvItems" 
                          runat="server"
                          AutoGenerateColumns="False" 
                          DataKeyNames="RmaDetailId" 
                          OnRowDataBound="gvItems_RowDataBound">
                  <Columns>
                          <asp:TemplateField HeaderText="Sku">
                               <ItemTemplate>
                                   <asp:Label ID="lblSku" runat="server" Text='<%# Eval("RmaSku") %>'>
                                   </asp:Label>
                               </ItemTemplate>
                          </asp:TemplateField>  
                          <asp:TemplateField HeaderText="Description">
                               <ItemTemplate>
                                   <asp:Label ID="lblName" runat="server" Text='<%# Eval("Name") %>'>
                                   </asp:Label>
                               </ItemTemplate>
                          </asp:TemplateField> 
                          <asp:TemplateField HeaderText="Serial Number">
                               <ItemTemplate>
                                   <asp:TextBox ID="txtSerialNum" runat="server" Text='<%# Eval("SerialNum") %>'>
                                   </asp:TextBox>
                                        <asp:RequiredFieldValidator ID="rfv1" 
                                                                    runat="server" 
                                                                    ControlToValidate="txtSerialNum" 
                                                                    ErrorMessage="*" 
                                                                    ForeColor="Red">
                                         </asp:RequiredFieldValidator>
                               </ItemTemplate>                                            
                          </asp:TemplateField>                                                                   
          </Columns>
      </asp:GridView>
  </td>
</tr>
<tr>
   <td>
       <asp:Label ID="lblErrorsExist1" runat="server" Font-Bold="true" Visible="false">Please Complete All Required Fields </asp:Label>
   </td>
</tr>       
<tr>
   <td>
       <asp:Button ID="btnSubmit" runat="server" Text="Submit" Font-Bold="True" ForeColor="Maroon" OnClick="btnSubmit_Click" />
   </td>
</tr>

Here is my C# code for the button click event:

protected void btnSubmit_Click(object sender, EventArgs e)
        {

                foreach (GridViewRow dr in gvItems.Rows)
                {
                    Label lRmaDetailId = dr.FindControl("lblRmaDetailId") as Label;
                    Label lSku = dr.FindControl("lblSku") as Label;
                    Label lSerReq = dr.FindControl("lblRmaSerialNumRequired") as Label;
                    TextBox sn = dr.FindControl("txtSerialNum") as TextBox;

                    Session["ErrorsExist"] = "";
                    lblErrorsExist1.Visible = false;

                    if (lSerReq.Text == "Y")
                    {
                        if (sn.Text.Length == 0)
                        {
                            lblErrorsExist1.Visible = true;
                            Session["ErrorsExist"] = "Y";
                        }
                        else
                        {
                            sn.BackColor = Color.White;
                        }
                    }
                }                    
        }

But when I enter a serial number value, the page displays the error message saying "Please Complete All Required Fields".  While in debug mode, sn.Text.Length is = 0.  But I enter a value like "XXXXXXX".  What am I doing wrong?