Particle insertion at once not possible with fix insert/rate/region

Dear all,
i guess i found a wrong information in the documentation regarding the fix insert/rate/region command.
The corresponding list of general_keywords and their parameters contains the following line:
"once = value to signalise that isertion takes place only once (the step after the fix is issued)"

The value "once", however, is not allowed with this command, as my bash output was:
"'insert_every' = once not allowed"

The doc page for fix insert/rate/region correctly states below:
"Insert particles into a granular run every few timesteps within a specified region at a specified rate." (in contrast to the fix insert/pack command - "Insert particles into a granular run either once or every few timesteps within the specified region, as defined via the region keyword.")

Regards,
sebastian