IE: Object Expected; Moz: Runs Fine

I have a html document, in which I include two standard libraries of
functions (supposed to help me with cross browser issues), and one
application-specific script file. All Javascript fcoz.

The very first attempt at invoking a function from this last script file
fails miserably in IE (6) but works as it should in Moz (1.4)

I have tried putting alert()s here and there, so I (think I) can tell
the exact spot where IE dies.

Structure is like this (all in HEAD):

<script type="text/javascript" src="../libPlatform.js"></script>
<script type="text/javascript" src="../libDOM.js"></script>
<script type="text/javascript" src="tooltip.js"></script>
<script type="text/javascript">

function register() {
initAPI();
alert('after init');
regTooltip('a1');
alert('after dreg-1');
regTooltip('a2');
}
</script>

initAPI sits in libPlatform, and executes. I get to see the first alert.
And then, boom, IE script errors on page: Object expected (points to
'regTooltip')

Am I missing something here? If my code is wrong, why is Mozilla so
forgiving about it? How can I persuade IE to get the function?

Note: if I put regTooltip() right above register(), so locally in the
document, it works.