diff --git a/apps/dav/lib/Migration/RemoveClassifiedEventActivity.php b/apps/dav/lib/Migration/RemoveClassifiedEventActivity.php
index 1f1edf285a..ad840d8100 100644
--- a/apps/dav/lib/Migration/RemoveClassifiedEventActivity.php
+++ b/apps/dav/lib/Migration/RemoveClassifiedEventActivity.php
@@ -75,6 +75,10 @@ class RemoveClassifiedEventActivity implements IRepairStep {
 		$result = $query->execute();
 
 		while ($row = $result->fetch()) {
+			if ($row['principaluri'] === null) {
+				continue;
+			}
+
 			$delete->setParameter('owner', $this->getPrincipal($row['principaluri']))
 				->setParameter('type', 'calendar')
 				->setParameter('calendar_id', $row['calendarid'])
@@ -105,6 +109,10 @@ class RemoveClassifiedEventActivity implements IRepairStep {
 		$result = $query->execute();
 
 		while ($row = $result->fetch()) {
+			if ($row['principaluri'] === null) {
+				continue;
+			}
+
 			$delete->setParameter('owner', $this->getPrincipal($row['principaluri']))
 				->setParameter('type', 'calendar')
 				->setParameter('calendar_id', $row['calendarid'])