Why am I being told that "indexOf" is not a function?

Here's my code:

function validateForm() {
	var dob = document.getElementById("usrDOB");
	var usrEEID = document.getElementById("usrEEID");
	var error = '';
	if(usrEEID.indexOf("S")==-1)
	{
		$('#usrEEID').css('border', 'solid 2px red').focus();
		error = "Make sure you include a capital \"S\" in the Employee ID field!";
	}
	if (!isValidDate(dob.value)) 
	{
		$('#usrDOB').css('border', 'solid 2px red').focus();
		error = "Date of Birth must be in the format XX/XX/XXXX\n\r";
	} 
	else 
	{
		$('#usrDOB').css('border', '');
	}
	
	if (error.length > 0) {
		alert(error);
		return false;
	}
	return true;
}

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Everything works except...

if(usrEEID.indexOf("S")==-1)
      {
            $('#usrEEID').css('border', 'solid 2px red').focus();
            error = "Make sure you include a capital \"S\" in the Employee ID field!";
      }

I keep getting an error that says "usrEEID.indexOf is not a function." What am I doing wrong?