Javascript code is not working

<form method='POST' action='' name='pay'>
		
		<label> Payment method:</label>
		
		 <select name='paymth' id='paymth' onChange='changetextbox();' >
      <option value='Deposit In Bank'>Deposit In Bank</option>
      <option value='Online'>Online Transfer (Net Banking/RTGS/NEFT/Mobile Banking)</option>
      <option value='Cheque Deposit In Bank'>Cheque Deposit In Bank</option>
    </select>
	
	<label >Name of Depositor*:</label>
	<input id='bd' type='text' name='depositor'>
	<label >Bank Name:</label>
	<input  type='text' name='bank'  required>
	<label >Date*: (DD-MM-YYYY)</label>
	<input  type='text' name='date' required>
	<label>Payment Proof*:(Scan copy, Deposit Receipt, Photo, Screenshot, Transaction Print. )</label>
	<br>
	<input type='file' name='proof' required>
	<input type='submit' name='payment' value='Submit'>
		</form>
		<script type="text/javascript">
function changetextbox()
{
    if (document.getElementById("paymth").value === "Online") {
        document.getElementById("bd").disable='true';
    } 
}
</script>