phpass gives error message when installing different module

When I installed a module, which had a dependancy on another (not yet installed) module, drupal displayed the page asking me if I wanted to install the latter module too. But it also showed an error message from phpass, which was totally unrelated to this process. I think this message should be shown when uninstalling this module.

Would you like to continue with enabling the above?. Some users passwords have already been converted. You can not disable this module. Please visit the user settings page, and revert to "Standard Drupal (md5)" so that the user passwords can be converted back.

I don't know if the error is with phpass or drupal core.