I have a checkbox in a form. When the user ticks the box, I want a string to be entered into text box in the same form. If they untick the box, I want the value to be cleared in the text box

Code:
<script language="javascript">
<!--

function inserthttp() {
    if (document.add_product.Multipage.value = 1) {
        document.add_product.MultipageURL.value = "some value"
    } else {
        document.add_product.MultipageURL.value = ""
    }
}

//-->
</script>

.........

<input type="checkbox" name="Multipage" value="1" onclick="javascript:inserthttp();">
This will insert the "some value" when you click on the checkbox, but it won't clear the value when you click on it again.

Any ideas?