Vista - Enabling commands with SelectedItems

I'm implementing an application in the DataModel-ViewModel-View
pattern and I'm having some trouble with commands bound to a listbox.
What I'd like to happen is for a given command to be enabled/disabled
based on the number of SelectedItems. Here's some of the relevant code
so far:

<StackPanel>
<Button
Command="{Binding Path=MyCommand}"
CommandParameter="{Binding Source=ListBox1,
Path=SelectedItems}"
>
Command
</Button>
<ListBox x:Name="ListBox1" ItemsSource="..."/>
</StackPanel>

In my code-behind I have a class derived from ICommand that implements
CanExecute() and Execute(). CanExecute() is called on creation of the
command and immediately before it is invoked by clicking the button,
but it is not called when SelectedItems changes.

I thought that CanExecute() would be called each time SelectedItems
changed and should enable/disable the button as appropriate. Am I
missing something?

Cheers,
Keith

Answered my own question: CanExecuteChanged must be called by the
ICommand implementation.

Answered my own question: CanExecuteChanged must be called by the
ICommand implementation.