strnatcasecmp() expects parameter 1 to be string, array given in (no replies)

When I want an overview of the visitors log the following thing happends.

Warning: strnatcasecmp() expects parameter 1 to be string, array given in
*****/public_html/statistieken/core/DataTable/Filter/Sort.php on line 140

Backtrace -->

#0 Piwik\Error::errorHandler(...) called at [:]
#1 strnatcasecmp(...) called at [/home/knelus/domains/hensens.eu/public_html/statistieken/core/DataTable/Filter/Sort.php:140]
#2 Piwik\DataTable\Filter\Sort->naturalSort(...) called at [:]
#3 usort(...) called at