'a' recognized as valid argument

[lukaszachy]

Allowed arguments are described in usage string as
Usage: did [last] [week|month|quarter|year] [opts]

I would expect failure for each unrecognised positional argument. But that is only partially true:
$ did b
ERROR Invalid argument: 'b'

$ did a
Status report for this week (2015-09-21 to 2015-09-27).
...

AFAIK there should be no positional argument which 'a' matches.