I am trying to use a dateFormatter with the following options:

{
    skeleton: 'yMMMM'
}

With locale en-US, it throws the following error:
E_UNSUPPORTED: Unsupported {"feature":"year pattern `u`"}.

It seems to be unhappy with the 4 M's; if I use any other number, then it works fine (but doesn't format it as desired). Based on the date symbol table, I would expect 4 M's to use the full month name. I'm trying to format a date as "Month Year", e.g. "September 2009" for en-us.

Is this user error, a bug, or a data issue?