SynchronizationContext is null in winforms

I'm creating a library which relies on capturing the SynchronizationContext in order to post callbacks back to the UI thread.

A user was having a weird condition where the callbacks were being posted to the ThreadPool instead. After investigating a bit, I came up with the following two test cases:

1:

public partial class Form1 : Form
{
    private Test test;

    public Form1()
    {
        test = new Test();
        InitializeComponent();
    }

    private class Test
    {
        public Test()
        {
            if (SynchronizationContext.Current == null)
                throw new InvalidOperationException("It's null! :(");
        }
    }
}

2:

public partial class Form1 : Form
{
    private Test test = new Test();

    public Form1()
    {
        InitializeComponent();
    }

    private class Test
    {
        public Test()
        {
            if (SynchronizationContext.Current == null)
                throw new InvalidOperationException("It's null! :(");
        }
    }
}

The 1st test runs fine, but the 2nd one throws an exception. Why?