diff --git a/lib/private/Collaboration/Reference/ReferenceManager.php b/lib/private/Collaboration/Reference/ReferenceManager.php
index 9287b66b2a230..1dad5415ed03f 100644
--- a/lib/private/Collaboration/Reference/ReferenceManager.php
+++ b/lib/private/Collaboration/Reference/ReferenceManager.php
@@ -172,7 +172,8 @@ public function invalidateCache(string $cachePrefix, ?string $cacheKey = null):
 			return;
 		}
 
-		$this->cache->remove(md5($cachePrefix) . '-' . md5($cacheKey));
+		// remove specific cache entry; using ($cacheKey ?? '') avoids md5(null) warnings and ensures stable hashing
+		$this->cache->remove(md5($cachePrefix) . '-' . md5($cacheKey ?? ''));
 	}
 
 	/**