Disable the button if the checkbox is unchecked

<script language="Javascript" type="text/javascript">
 function enable(){
  if (document.agreement_form.agreed.checked==''){
   document.agreement_form.submit_button.disabled=true
  }else{
   document.agreement_form.submit_button.disabled=false
  }
 }
</script>

.....
 <form name="agreement_form">
 <input type="checkbox" name="agreed" onchange="enable()">Do you want to test ?
 <br>
 <input type="submit" value="Submit" disabled name="submit_button">