Error in xml file using the XmlSerializer

I have an website with a administrator page. On this page a user is able to create, update or delete employees. I want to save these employees in a xml file using the XmlSerializer of C#.

The employee class looks like this:

public class Employee
{
    public int id { get; set; }
    public String lastname { get; set; }
    public String firstname { get; set; }
    public String enterdate { get; set; }
    public int salarylevel { get; set; }
}

However, I get an error while reading the xml file and do not know what to do:

In System.InvalidOperationException is an exception of type "System.Xml.dll" occurred, but this was not in the user code processed.

Additional information: error in XML document (0,0).

_{Translated from german.}

I created the project as an empty WebAPI and I have an EmployeeController that looks like this:

public class EmployeeController : ApiController
{
    List<Employee> employeelist = new List<Employee>();

    [HttpGet]
    public IEnumerable<Employee> getEmployees()
    {
        XmlSerializer serializer = new XmlSerializer(typeof(List<Employee>));
        FileStream stream = new FileStream(System.Web.Hosting.HostingEnvironment.ApplicationPhysicalPath + "employees.xml", FileMode.Open);
        employeelist = (List<Employee>)serializer.Deserialize(stream);

        return employeelist;
    }
}

The error occurs at: employeelist = (List<Employee>)serializer.Deserialize(stream);

The controller gets called with JQuery and the getJSON method using the url http://localhost:5267/api/employee/. All employees should be saved in a xml file that is called employees.xml which contains only one line at the moment:

<?xml version="1.0" encoding="utf-8" ?>

What I expected to happen was, that I get an empty list because there are no related xml elements in the file and therefore the page shows no employees.

I guess that I messed it up because I created the xml file manually and without the serializer or that the serializer cannot handle the one line xml. Does someone know how I should solve this problem?