Api call VisitsSummary.getUniqueVisitors with period range not working

Hi I’m trying to make this API call:

http://www.XXXX/index.php?module=API&method=VisitsSummary.getUniqueVisitors&idSite=XX&period=range&date=XXXX-XX-XX,XXXX-XX-XX&format=xml&token_auth=XXXXXX

I try with diferent ranges and returns 0…
An example with the demo:

http://demo.piwik.org/index.php?module=API&method=VisitsSummary.getUniqueVisitors&idSite=7&period=range&date=2011-10-20,2011-10-21&format=xml

If you try, it will return 0 when it should return the sum of this:

http://demo.piwik.org/index.php?module=API&method=VisitsSummary.getUniqueVisitors&idSite=7&period=day&date=2011-10-20&format=xml

and this:

http://demo.piwik.org/index.php?module=API&method=VisitsSummary.getUniqueVisitors&idSite=7&period=day&date=2011-10-21&format=xml

Help me!

Unique visitors are disabled by default for range, but you can enable it: How to - Analytics Platform - Matomo

Thanks Matt, I solved with that.