Rozdilne chovani IE x Firefox pri pouzivani input image

element <input type=image name=tlacitko...>
ve firefoxu vse funguje bez problemu, ale IE nedokaze ziskat $_REQUEST["tlacitko"], je pro nej NULL

<?php
echo "Proc Firefox vypise \"ok\" IE nikoliv:<br>".$_REQUEST[zmenit];

?>
<form action="<?echo $_SERVER['PHP_SELF'];?>" method="post">
<table>
<tr>
<td>Vypis:</td><td><input type="image" src="imgs/zmenit.gif" alt="zmenit" name="zmenit" value="ok"></td>
</tr>
</table>
</form>