ASP.NET "OnTextChanged" Error

Question

Hello,

I am new to Web technology and ASP.NET. When I was practicing I had to use the following code.

<asp:TextBox ID="txtFontSize" OnTextChanged="ControlChanged" AutoPostBack="true" runat="server"/>

but when the web program runs, when I type something in the mentioned textbox I get the following error.

"Microsoft JScript runtime error: The value of the property 'WebForm_TextBoxKeyHandler' is null or undefined, not a Function object"

When I try it with a multilined textbox the code it working perfectly. Can someone please help me?
Thank you in Advance