Imagebutton event fired when enter key pressed in a single line textbox

I have a usercontrol that is acting as a header, in the control I have
a number of imagebuttons. My problem is that when a user is typing in a
textbox control on the main page and hit the enter key the first image
button in my Header usercontrole onclick event is fired. This button
clears my session variables and returns the user to the home page. The
user then clicks the back button and gets an error on the next button
on the page they click because the expected session variables are not
there.
To resolve this I have placed a blank imagebutton in front of the
first button and hidden it. This works, but can some one explain to me
the purpose of this and if there is perhaps a better method of handling
this situation.

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