Adding existing models to many-to-many relationship with Entity Framework 5

Question

I have a table, Modules and a table Students, that have a many-to-many relationship through a pivot table ModuleStudents. I am trying to add students to a module but encounter an error:

An unhandled exception of type 'System.NullReferenceException' occurred in TestingEF.exe

Additional information: Object reference not set to an instance of an object.

SchoolContext context = new SchoolContext();

List<Module> modules = context.Modules.ToList();
Student student = context.Students.Single(s => s.MatriculationNumber == 40122047);

foreach (Module module in modules)
{
    module.Students.Add(student);
}

context.SaveChanges();

What am I doing wrong here? I am fetching all the modules in the database and returning them as a list which I am then iterating over and trying to add a student to? I don't understand why I am receiving a System.NullReferenceException

Contents of the output from the excpetion: https://gist.github.com/anonymous/bb719a215ded5518af46

I am using Code First for this project (a learning exercise using a Console Solution).

Module class:

class Module
{
    public int ModuleID { get; set; }
    public string Code { get; set; }
    public string Name { get; set; }

    public virtual Course Course { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}

Student class:

class Student
{
    public int StudentID { get; set; }
    public int MatriculationNumber { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }

    public virtual Course Course { get; set; }
    public virtual ICollection<Module> Modules { get; set; }
}

Answer 1

Try changing your Module class creating the Students list in the default constructor, like this:

class Module
{
    public Module()
    {
        this.Students = new List<Student>();
    }

    public int ModuleID { get; set; }
    public string Code { get; set; }
    public string Name { get; set; }

    public virtual Course Course { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}

Answer 2

A call stack for the exception would be nice but if I had to make a wild guess, I would say that Students is null in this line module.Students.Add(student);. Are you using code first to build your models? If so then you can initialize Students in Module's constructor; like so:

public Module
{
    Students = new List<Student>();
}