Cancel box error

I have several dropdown boxes and cancel box. If user did not make a selection from dropdown box, instead directly click cancel box. I got the error:
GID had a selectedIndex which is invalid because it does not exist in the list of theItems. Parameter name: value
 Here is my code:
 protected void Cancel_Click(object sender, EventArgs e)
    {
            PID.SelectedIndex = 0;
            GID.SelectedIndex = 0;
            Tid.SelectedIndex = 0;
            sid.SelectedIndex = 0;
}
 How to prevent this error?
 Thank you very much.
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