preg_match() error just started appearing on my site

the following error just began appearing on my site:

Warning: preg_match(): Compilation failed: nothing to repeat at offset 0 in /home/oplaws/public_html/wp-content/plugins/membership/membershipincludes/includes/functions.php on line 1013

Any ideas? Would like to resolve.

Additional info:

disappears when membership plugin is deactivated.