"image-flush all" not recognized

Using Drupal 7.15 and Drush 5.7 on a Windows system, the command "drush image-flush" works as expected, to flush all caches:

Choose a style to flush.
 [0]  :  Cancel
 [1]  :  all
 [2]  :  thumbnail
 [3]  :  medium
 [4]  :  large
1
All image styles flushed                                               [success]

Specifying an image style, e.g., "drush image-flush thumbnail", also works.

But "drush image-flush all" does not:

Image style all not recognized.                                          [error]

Perhaps Drush is erroneously searching for "all" as the name of an image style, instead of processing all of the existing ones.