How to return http error status code along with custom message in a class

I am wanting to return an https error status code with a custom message in a class. However, whenever I try to do so it says it is unable to convert from IResult to my model class employee. Prior to creating a class that handles the logic for sql I had it as the following in the program.cs file which worked as expected when I entered a value that did not exist in the db.

app.MapGet("/employee/{id}", (ApplicationDbContext db, int id) =>
{
    var employee = db.Employee.Find(id);
    if (employee == null)
        return Results.NotFound("Employee not found");
    return Results.Ok(employee);        
});

Now creating a separate class to have all the logic in one file I have the following in the program.cs file and the logic in the separate file.

app.MapGet("/employee/{id}", (IDataRepository dr, int id) =>
{
    return dr.GetEmployee(id);
});

now in the separate file:

 public Employee GetEmployee(int id)
        {
            var employee = _db.Employee.Find(id);
            if (employee == null)
            {
                return Results.NotFound("Employee not found!");
            }
            return employee;
        }

It suggested a cast to my model class, employee. When I enter an id knowing that it does not exist in the db it gives me a casting error saying cannot convert the two. Any suggestions would be greatly appreciated.