Can't add Watchers to an issue when there is a space in username

On the ManageWatchers page, adding a username which contains a space does not work. The username is treated as two separate usernames.

eg.
When adding "fred bloggs, joe smith" the following is displayed when 'Add' is clicked.

Errors:
The user "fred" could not be found. This user will not be added to the watch list.
The user "bloggs" could not be found. This user will not be added to the watch list.
The user "joe" could not be found. This user will not be added to the watch list.
The user "smith" could not be found. This user will not be added to the watch list.