Subdomain login error: "incorrect or out of date key"

Using Domain Mapping plugin and the "Cross-domain auto login" option causes a 500 error in the login process in a subdomain.

Error message:
PHP Fatal error: Call to undefined method Domainmap_Module_Cdsso::is_subdomain() in /home/wordpress/wp-content/plugins/domain-mapping/classes/Domainmap/Module/Cdsso.php on line 227