[ic] Javascript error with Place Order button.

[.]

When trying to submit an order using this button:

        [button
                name="mv_click"
                src="__IMAGE_DIR__/continue_on.gif"
                text="Place Order"
                hidetext=1
                form=checkout
        ]
                mv_todo=submit
        [/button]

it is returning a javascript error as follows:

Error: 'checkout.mv_click_Place_Order' is null or not an object
Code: 0

The funny thing is, the javascript source being generated is *precisely* the
same as for another page with exactly the same [button] tag, all bar the
actual image name. That other page works, this one doesn't. Anyone have any
ideas?