last1 works with day and week, but does not work with month and year – it gives last 2 entries, not 1.

Example with week (works correctly):
http://piwik.org/demo/?module=API&method=Referers.getKeywords&idSite=1&period=week&date=last1&format=php&filter_limit=5&prettyDisplay=1

Example with month (works incorrectly):
http://piwik.org/demo/?module=API&method=Referers.getKeywords&idSite=1&period=month&date=last1&format=php&filter_limit=5&prettyDisplay=1

From user point of view I’m asking 1 entry – so there will be only 1 entry – even if we know that month and year are incomplete.
Keywords: last