C#: Checking if an object exists in a list

Question

I have a Subject class and a Student class which contains a list of Subjects implemented as:

class Student
{
    // 1 Properties and Fields
    public List<Subject> Subjects;

    // 2 Constructor
    public Student()
    {
        Subjects = new List<Subject>();
    }

    // 3 Methods
    public Subject SelectSubject(Subject subject)
    {
        if (Subjects.Contains(subject))
        {
            return subject;
        }
        return null;
    }
}

In the SelectSubject method, I want to pass in a subject and check if that variable exists in the Subjects class. However, I am getting a

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Can someone please help me fix this?