theme system fails to find themes if a directory named '0' is present in the themes directory

Steps to reproduce:
1) place a number of themes in sites/all/themes/custom
2) look at "http:/example.com/admin/build/themes". The custom themes should all be listed
3) create a directory named '0' (the digit 0) in the directory of one of those themes
3) look at "http:/example.com/admin/build/themes". One or more of the themes have disappeared from the list

Platform: xamp on Vista, drupal 6.2, only core modules

I assume that the directory name "0" may be evaluated as false somewhere in the code causing the file scanning to halt.