runtime: Immutable hash-based collections do not accept `null` values

I have spent two days chasing down an evil bug. I narrowed down the problem with this test case demonstrating different behavior for different collection classes that implement the same interface. Specifically Contains(null) throws a ArgumentNullException for ImmutableSortedSet but not for Array or the mutable SortedSet.

    using System.Collections.Generic;
    using System.Collections.Immutable;
    using Xunit;

    public class SortedSetSpec
    {
        [Fact]
        public void WTFNullCheck()
        {
            var data = new[] {"a", "b", "c", "d"};

            SortedSet<string> ss = new SortedSet<string>(data);
            ImmutableSortedSet<string> iss = ss.ToImmutableSortedSet();

            // This passes
            data.Contains(null).Should().BeFalse();

            // This passes
            ss.Contains(null).Should().BeFalse();

            // This throws an exception
            iss.Contains(null).Should().BeFalse();
        }
    }

Given that all classes Array / SortedSet and ImmutableSortedSet implement ICollection<T> shouldn’t they have the same behavior on the Contains(null) call?

The bug manifested itself when I data bound an ImmutableSortedSet to the ItemsSource property of a listbox.

     <ListBox x:Name="ListBox" Margin="2" 
              ItemsSource="{Binding WorkflowTags}" 
              SelectedItem="{Binding SelectedTag}" >
     </ListBox>

The problem is that at some point deep in the databinding code ListBox (aka Selector ) asks the collection Contains(null)?? and then it crashes if I’ve databound an ImmutableSortedSet.

So is this a bug with ImmutableCollections or with WPF or is it expected behavior and I should know better than to use ImmutableSortedSet?

[EDIT] Add syntax highlighting by @karelz