My form does not work well in IE7 - help please

I have this page:

Open this page on IE7

http://www.cantinho.org/admin/cantinho/euseilapa.php

Type a value that return nothing from the database: ie. sskdjgskdjgksdjgskgj

Click (with the mouse) the submit button.

Result:
A message saying record not found appears.




Now do the same, but instead of CLICK with the mouse, press enter with the
keyboard.
No "record not found" (in portuguese) message appears.


Question:
Why? And how can I fix this?





Thanks in advance
ps- in Firefox all works fine.

Solution:

I need to add a hidden field. Why? I have no Idea.

<input type="hidden" name="pesquisa" id="pesquisa" value="Pesquisar" />
<input type="submit" value="Pesquisa O BiXo"/>

Thanks anyway.