Subclass of Walker_Category_Checklist

I’m making a walker subclass similar to Walker_Category_Checklist, but I get an error when I inherit from Walker_Category_Checklist:

Fatal error: Class ‘Walker_Category_Checklist’ not found

I don’t get an error when I inherit from Walker, copy the Walker_Category_Checklist code, & modify it.

I’m adding my walker code in a theme’s functions.php file, and not within any action or filter.

Where should I be adding this code so it is executed after the Walker subclasses (such as Walker_Category_Checklist) are defined?

Thanks,
Dan