Value cannot be null.Parameter name: source

Endpoint throwing argument null exception as:

Exception:
System.ArgumentNullException: Value cannot be null.
Parameter name: source
   at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)
   at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)

When I investigated then found that it's coming from Linq extension method:

_service.GetDoc().FirstOrDefault();

Where GetDoc() as:

 public IEnumerable<Doc> GetDoc()
 {
    //
 }