Status being set to 0 unless theme is defined as default theme

Perhaps this is a Drupal issue, but I've found that when I install this theme on a site with other enabled themes, and try to configure the theme and/or administer blocks for the theme, I get "access denied"... unless Clean is the default theme for the whole system. I've tried poking around in theme.inc and block.module, but I can't seem to figure it out. The status flag appears to be set properly when list_themes first builds up its static list, but then, sometime between that and the time the menu system takes a look at the theme, the flag is flipped off. I've tried clearing out the cache a couple of times, but that doesn't seem to help.