"Submit is not a function" error in JavaScript

Can anyone tell me what is going wrong with this code? I tried to submit a form with JavaScript, but an error ".submit is not a function" shown. See below for more details of the code:

<form action="product.php" method="get" name="frmProduct" id="frmProduct" enctype="multipart/form-data">

<input onclick="submitAction()" id="submit_value" type="button" name="submit_value" value="">

</form>

<script type="text/javascript">
    function submitAction()
    {
        document.frmProduct.submit();
    }
</script>

I also tried this:

<script type="text/javascript">
    function submitAction()
    {
        document.forms["frmProduct"].submit();
    }
</script>

Both show me the same error :(