Need Help With Sqldatasource And Updating A Field In Sql 2005

I am experiencing some wacky errors here. While trying to update a field that does not allow nulls and the default value is set to '', I keep receiving an exception error that:

 Cannot insert the value NULL into column 'image_name', table 'DB_123871.dbo.tWebBlogs'; column does not allow nulls. UPDATE fails. The statement has been terminated.

 <asp:GridView ID="GridView1" runat="server" AllowPaging="True" AllowSorting="True" DataSourceID="SqlGetBlogs" DataKeyNames="article_id" AutoGenerateColumns="False" CssClass="GridView" CellPadding="4" HorizontalAlign="Center" Width="875px">
<Columns>
<asp:CommandField CancelImageUrl="~/images/Cancel.gif" EditImageUrl="~/images/Edit.gif"
UpdateImageUrl="~/images/Update.gif" ButtonType="Image" HeaderText="Edit" ShowEditButton="True">
</asp:CommandField>
<asp:BoundField DataField="article_id" HeaderText="ID" ReadOnly="True" />
<asp:TemplateField HeaderText="Artilce Header">
<EditItemTemplate>
<asp:TextBox ID="ArticleHeaderTxt" runat="server" Text='<%# Bind("article_header") %>' Width="250"></asp:TextBox>
<asp:RequiredFieldValidator ID="ArticleHeaderTxtReq" runat="server" ControlToValidate="ArticleHeaderTxt" Display="Dynamic" EnableClientScript="true" ErrorMessage="Article Header Required" SetFocusOnError="true"></asp:RequiredFieldValidator>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="ArticleHeaderLbl" runat="server" Text='<%# Eval("article_header") %>' ></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Article Description">
<EditItemTemplate>
<asp:TextBox ID="ArticleDescriptionTxt" runat="server" Text='<%# Bind("article_description") %>' Width="325" Rows="8" TextMode="MultiLine"></asp:TextBox>
<asp:RequiredFieldValidator ID="ArticleDescTxtReq" runat="server" ControlToValidate="ArticleDescriptionTxt" Display="Dynamic" EnableClientScript="true" ErrorMessage="Article Description Required" SetFocusOnError="true"></asp:RequiredFieldValidator>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="ArticleDescriptionLbl" runat="server" Text='<%# Eval("article_description") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Link Short Text">
<EditItemTemplate>
<asp:TextBox ID="ArticleLinkTxt" runat="server" Text='<%# Bind("short_link_text") %>'></asp:TextBox>
<asp:RequiredFieldValidator ID="ArticleLinkTxtReq" runat="server" ControlToValidate="ArticleLinkTxt" Display="Dynamic" EnableClientScript="true" ErrorMessage="Article Link Text Required" SetFocusOnError="true"></asp:RequiredFieldValidator>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="ArticleLinkLbl" runat="server" Text='<%# Eval("short_link_text") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Article Link">
<EditItemTemplate>
<asp:TextBox ID="ArticleLink" runat="server" Text='<%# Bind("article_link") %>' Width="250"></asp:TextBox>
<asp:RequiredFieldValidator ID="ArticleLinkReq" runat="server" ControlToValidate="ArticleLink" Display="Dynamic" EnableClientScript="true" ErrorMessage="Article Link URL Required" SetFocusOnError="true"></asp:RequiredFieldValidator>
</EditItemTemplate>
<ItemTemplate>
<asp:HyperLink ID="ArticleLinkLnk" runat="server" CssClass="LinkNormal" NavigateUrl='<%# Eval("article_link") %>'
Target="_blank" Text="View Link"></asp:HyperLink>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Image Name">
<EditItemTemplate>
<asp:TextBox ID="Image1Txt" runat="server" Text='<%# Bind("image_name") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Image ID="Image1Img" runat="server" AlternateText='<%# Eval("image_name") %>' ImageUrl='<%# Eval("image_name", "~/Blogs/Images/Thumbs/{0}") %>' />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Date Entered">
<EditItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Eval("article_date_entered", "{0:d}") %>'></asp:Label>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Eval("article_date_entered", "{0:d}") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Active?">
<EditItemTemplate>
<asp:CheckBox ID="ActiveChk" runat="server" Checked='<%# Bind("active") %>' />
</EditItemTemplate>
<ItemTemplate>
<asp:CheckBox ID="ActiveChk" runat="server" Checked='<%# Eval("active") %>' Enabled="false" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Delete?">
<ItemTemplate>
<asp:ImageButton ID="DeleteBtn" runat="server" AlternateText="Delete Record" CommandName="Delete"
ImageUrl="~/images/Delete.gif" OnClientClick="return confirm('Are you sure you want to delete this blog?');" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
<RowStyle HorizontalAlign="Left"></RowStyle>
<EditRowStyle Font-Bold="False"></EditRowStyle>
<HeaderStyle CssClass="GridViewHeader" HorizontalAlign="Left"></HeaderStyle>
<AlternatingRowStyle CssClass="GridViewAltRow"></AlternatingRowStyle>
</asp:GridView>
</td>
</tr>
<tr>
<td>
<div style="text-align: center;"><asp:Label ID="recordset_lbl" runat="server" CssClass="HelpTextNormal"></asp:Label></div>
</td>
</tr>
</table>
</div>

<asp:SqlDataSource ID="SqlGetBlogs" runat="server" ConnectionString="<%$ connectionStrings:dbconn1 %>"
ProviderName="System.Data.SqlClient"
SelectCommand="SELECT article_id, article_header, article_description, image_name, short_link_text, article_link, article_date_entered, active FROM tWebBlogs ORDER BY article_date_entered DESC"
SelectCommandType="Text"
UpdateCommand="UPDATE tWebBlogs SET article_header=@article_header, article_description=@article_description, image_name=@image_name, short_link_text=@short_link_text, article_link=@article_link, active=@active WHERE article_id=@article_id"
UpdateCommandType="Text"
DeleteCommandType="Text"
DeleteCommand="DELETE tWebBlogs WHERE article_id=@article_id">
<DeleteParameters>
<asp:Parameter Name="article_id" />
</DeleteParameters>
</asp:SqlDataSource> 

 Please help! I am stumped!