diff --git a/spec/problems.schema.json b/spec/problems.schema.json
index c82502ad4727cf777827d64c88868ed7aef78d6f..1887e93f5cd1e91b74928d1e9de1ed9e1cd5b5ad 100644
--- a/spec/problems.schema.json
+++ b/spec/problems.schema.json
@@ -21,7 +21,7 @@
       "instance": {
         "description": "Betroffener Teil der Übertragung. Mögliche Werte: `submission`, `reply`, `metadata`, `data`, `attachment:` + UUID des Attachments oder `other`",
         "type": "string",
-        "pattern": "^(submission|reply|metadata|data|attachment:.+|other)$"
+        "pattern": "^(data|metadata|attachment:[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[4][0-9A-Fa-f]{3}-[89AB][0-9A-Fa-f]{3}-[0-9A-Fa-f]{12}|reply|submission|other)$"
       }
     },
     "required": [