Membership incompatible with XMLSitemap


I just installed Membership and it seems to be very incompatible with XMLSitemaps (XML Sitemap Generator for WordPress 4.0beta4) which is rather important nowadays.

See the error I get:

Warning: Invalid argument supplied for foreach() in /home/tapuat5/public_html/ on line 1198

The XMLSitemap does work if I uninstall the membership plugin.

Could you tell me what to do ?