Value cannot be null exception

I am getting the following exception when in my code, I called on my ShellObjectBase derived class when the column header is tapped.

RaiseChangedEvent(new ShellObjectChangedEventArgs(ShellObjectChangeKind.SortRequested, null));

System.ArgumentNullException
HResult=0x80004003
Message=Value cannot be null.
Parameter name: key
Source=mscorlib
StackTrace:
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at ActiproSoftware.Windows.Controls.Shell.ShellObjectItemAdapter.SortChildren(TreeListBox ownerControl, ShellObjectViewModel parentViewModel, Boolean isReversingSortDirection)
at ActiproSoftware.Windows.Controls.Shell.ShellListView.OnViewModelModelChanged(Object sender, ShellObjectChangedEventArgs e)
at ActiproSoftware.Windows.Controls.Shell.ShellObjectViewModel.#N6j(Object #xhb, ShellObjectChangedEventArgs #yhb)
at ActiproSoftware.Windows.Controls.Shell.ShellObjectViewModel.#L6j(Object #xhb, ShellObjectChangedEventArgs #yhb)
at System.EventHandler`1.Invoke(Object sender, TEventArgs e)
at ActiproSoftware.Shell.ShellObjectBase.RaiseChangedEvent(ShellObjectChangedEventArgs e)

It is the key being null in the call to Dictionary`2.Insert. What is the key there and any idea why it is null?

Thanks