Modules setting $classes in their preprocess functions breaks expected Zen classes

While the $classes variable is specific to Zen in Drupal 6, it would still be nice if modules could set $classes in their preprocess functions and not have Zen fail to add the $hook class to the template.

For example, if foo.module adds a string to $classes in foo_preprocess_block(), then Zen will use the module's value, but fail to add the "block" class to its final $classes variable.