Disable textfield with checkbox

Hey guys and gals,
I am pretty new to javascript so please bear with me.

I have a checkbox that when clicked, I would like to disable a textfield.

Here is my function:

<script type="text/javascript">
function disableForm()
{
	if (document.getElementById("fShipping").checked)
	{
		document.getElementById("freeLimit").disabled=true;
	}
}	
</script>

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And here is my HTML:

<p id="freeShipping">
  <input name="fShipping" id="fShipping" type="checkbox" value="1" onFocus="disableForm()"/> 
  Free shippping on all orders over $
  <input name="freeLimit" id="freelimit" type="text" size="6" maxlength="6" />
</p>

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Any ideas would be greatly appreciated!

Thanks,
Slim