"object Doesn't Support This Property Or Method" Error

I have a function that loops through 2 select lists and records the
values of any hi-lighted options a user would have selected. It then
sets 2 corresponding "hidden" form elements to the values and submits
the form data to the server.

I was error free until I added the following line:

document.myform.submit();

This produces the "object doesn't support this property or method"
error. The troubleshooting steps I have taken are:

1) Confirmed the existance of <form> and </form> tags.
2) Confirmed there is no typo in the form name.
3) Tried changing the form name from "bhd2pib_form" to just "myform".
4) Commented out the suspected offending line of code and I am again
error free, uncomment it, the error comes back.
5) Added a normal "submit" button to test the action and post
attributes of the <form> tag and that works fine.

I am having a hard time believing a simple method call such as
document.myform.submit(); is so difficult to figure out! Please, any
help would be greatly appreciated. The function is pasted below in
it's entirety:

<script type="text/javascript">
function setValues()
{
var bhd_list = document.myform.bhd_headings;
var pib_list = document.myform.pib_headings;

var bhd_values = '' // String of selected base_heading ids to
map
var pib_value;

for ( i=1; i<bhd_list.options.length; i++ ) {
if ( bhd_list.options[i].selected ) {
if ( !bhd_values ) {
bhd_values = bhd_values +
bhd_list.options[i].value;
}
else {
bhd_values = bhd_values + ':' +
bhd_list.options[i].value;
}
}
}

for ( j=1; j<pib_list.options.length; j++ ) {
if ( pib_list.options[j].selected ) {
pib_value = pib_list.options[j].value;
break;
}
}

if ( !bhd_values ) {
alert( "You have to select at least 1 base heading to
map or else this page doesn't make a lot of sense to have." );
}

if ( !pib_value ) {
alert( "You have to select a PIB heading to map to" );
}

// Set values of hidden fields
document.myform.bhd_values.value = bhd_values;
document.myform.pib_value.value = pib_value;

document.myform.submit(); //this line causes the error
}
</script>