I'm using the latest Piwik 2.16.1. If I try to export website statistics (referrers) with expanded data, the following error appears:
Error: Invalid dimension 'null'.

My URL looks like this:
https://domain.com/piwik/index.php?module=API&method=Referrers.getWebsites&format=TSV&idSite=9&period=range&date=2016-04-01,2016-04-30&pivotBy=null&expanded=1&translateColumnNames=1&language=de
URL shortened for privacy.

Removing pivotBy=null from the URL manually fixes this problem. I'm not sure why it should be inside the URL, must be generated by the widget.