diff --git a/lib/private/Share20/DefaultShareProvider.php b/lib/private/Share20/DefaultShareProvider.php
index a9a5bd7903133..4ffa2dc033552 100644
--- a/lib/private/Share20/DefaultShareProvider.php
+++ b/lib/private/Share20/DefaultShareProvider.php
@@ -329,6 +329,10 @@ public function acceptShare(IShare $share, string $recipient): IShare {
 				throw new ProviderException('Group "' . $share->getSharedWith() . '" does not exist');
 			}
 
+			if (is_null($user)) {
+				throw new ProviderException('User "' . $recipient . '" does not exist');
+			}
+
 			if (!$group->inGroup($user)) {
 				throw new ProviderException('Recipient not in receiving group');
 			}