check box checked javascript

I am trying to set a value in a text box based on whether the checkbox is checked or not The function is attached to onclick of the checkbox This simple thing is not working :(

<div>
MyCheckbox
<input type="checkbox" id="Check1" name="FirstCkName" 
 onclick="testCheckbox(Check1,TextBx1)" />
 </div><br>

 <div>
  CheckBx Text Box
  <input id="TextBx1" name="CheckBxName"  type="text"  /> 
  </div><br>

<script>
function testCheckbox(oCheckbox,oTxtbox)
{

if (oCheckbox.checked == true)
{
    document.getElementById("oTxtbox").value=1;
}
else
{
    document.getElementById("oTxtbox").value="";
}
}
</script>

Link to JSfiddle https://jsfiddle.net/JS_learner/2750639m/30/