self referencing loop detected for property dotnet core code example

Example: asp.net core api Self referencing loop detected for property

public class StaffMember
{
    public string FirstName { get; set; }
    public virtual Department Department { get; set; }
}

public class Department
{
    public string DepartmentName { get; set; }
    [JsonIgnore]
    public virtual ICollection StaffMembers { get; set; }
}