Name 'txtauthor' is not declared

Hi,
On my aspx page I have a code;
<asp:TextBox ID="txtauthor" runat="server" Cssclass="field" ReadOnly = "True"></asp:TextBox> On Page load, I have;
txtauthor.Text = cmbchngreq.Text
I receive the error;
Name: 'txtauthor' is not declared..
Why do I receive this error = at runtime txtauthor.Text = cmbchngreq.Text
and the txtauthor.text takes the value from cmbchngreq.Text
Thanks 

 
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