Getting Error " 'ASP.default_aspx' does not contain a definition for ..."

Hello,
I am working on the following code snippet in C#:
<asp:CheckBox ID="giftNo" runat="server" Checked = "true" oncheckedchanged="giftNoChecked"/>
And here's the corresponding event in the code-behind (yes, the event is currently blank):
protected void giftNoChecked(object sender, EventArgs e)         {         }
When I try to run my code, this snippet causes the error: " 'ASP.default_aspx' does not contain a definition for 'giftNoChecked' and no extension method 'giftNoChecked' accepting a first argument of type 'ASP.default_aspx' could be found".
I have setup the event using Visual Studio's Properties window, so I know the event declaration and ...