Mailman: Can't add member to list via Webmin

While attempting to manually add a member to a Mailman list, I get this error. Any ideas?

Failed to add list member : Add members to a list from the command line. Usage: add_members [options] listname Options: --regular-members-file=file
-r file A file containing addresses of the members to be added, one address per line. This list of people become non-digest
members. If file is -', read addresses from stdin. Note that -n/--non-digest-members-file are deprecated synonyms for this option. --digest-members-file=file -d file Similar to above, but these people become digest members. --welcome-msg= -w Set whether or not to send the list members a welcome message, overriding whatever the list's send_welcome_msg’ setting is. --admin-notify= -a Set whether or not to send the list administrators
a notification on the success/failure of these subscriptions, overriding whatever the list’s admin_notify_mchanges' setting is. --nomail -n Set the newly added members mail delivery to disabled by admin. --help -h Print this help message and exit. listname The name of the Mailman list you are adding members to. It must already exist. You must supply at least one of -r and -d options. At most one of the files can be -’.