diff --git a/lib/private/Collaboration/Reference/ReferenceManager.php b/lib/private/Collaboration/Reference/ReferenceManager.php
index a837651f1fec0..06f7ccc66bb5c 100644
--- a/lib/private/Collaboration/Reference/ReferenceManager.php
+++ b/lib/private/Collaboration/Reference/ReferenceManager.php
@@ -174,7 +174,7 @@ public function invalidateCache(string $cachePrefix, ?string $cacheKey = null):
 			return;
 		}
 
-		$this->cache->remove(md5($cachePrefix) . '-' . md5($cacheKey));
+		$this->cache->remove(md5($cachePrefix) . '-' . md5($cacheKey ?? ''));
 	}
 
 	/**