Exception thrown during upgrade: null

Getting the following error when trying to upgrade from 2.5.1 to 2.6.1.

Exception thrown during upgrade: null
java.lang.NullPointerException
at com.atlassian.core.user.GroupUtils.existsGroup(GroupUtils.java:91)
at com.atlassian.jira.upgrade.tasks.UpgradeTask_Build60.doUpgrade(UpgradeTask_Build60.java:41)
at com.atlassian.jira.upgrade.UpgradeManagerImpl.doUpgradeTaskSucess(UpgradeManagerImpl.java:584)
at com.atlassian.jira.upgrade.UpgradeManagerImpl.doUpgrade(UpgradeManagerImpl.java:335)
at com.atlassian.jira.upgrade.UpgradeManagerImpl.doUpgradeIfNeeded(UpgradeManagerImpl.java:281)
at com.atlassian.jira.upgrade.UpgradeManagerImpl.doUpgradeIfNeededAndAllowed(UpgradeManagerImpl.java:241)
at com.atlassian.jira.upgrade.UpgradeLauncher.contextInitialized(UpgradeLauncher.java:83)
at com.caucho.server.http.Application.init(Application.java:1832)
at com.caucho.server.http.VirtualHost.initWars(VirtualHost.java:824)
at com.caucho.server.http.VirtualHost.init(VirtualHost.java:687)
at com.caucho.server.http.ServletServer.initHosts(ServletServer.java:853)
at com.caucho.server.http.ServletServer.initInternal(ServletServer.java:698)
at com.caucho.server.http.ServletServer.init(ServletServer.java:509)
at com.caucho.server.http.ResinServer.init(ResinServer.java:382)
at com.caucho.server.http.ResinServer.main(ResinServer.java:1142)
at com.caucho.server.http.HttpServer.main(HttpServer.java:103)