Pressing Enter on a textbox results in a javascript error

The scenario is this: You use a ScriptManager, a TextBox and a Button and your project is at least referencing the Ajax Control Tookit library. You expect when pressing Enter in the TextBox to get to the button Click event. Instead you get the Microsoft JScript runtime error: ‘this._postBackSettings.async’ is null or not an object javascript error.

I found the solution here: JScript Exception in AJAX Control Toolkit. Basically, put the textbox and the button in a Panel and set the DefaultButton property to the button id.