diff --git a/artiq/examples/no_hardware/repository/hdf5_attributes.py b/artiq/examples/no_hardware/repository/hdf5_attributes.py
index 8237435569..f216a64b32 100644
--- a/artiq/examples/no_hardware/repository/hdf5_attributes.py
+++ b/artiq/examples/no_hardware/repository/hdf5_attributes.py
@@ -19,3 +19,6 @@ def run(self):
         self.set_dataset("dummy", dummy,
                          broadcast=True, archive=True)
         self.set_dataset_metadata("dummy", {"k1": "v1", "k2": "v2"})
+
+        # Attach metadata to an absent key is no-op
+        self.set_dataset_metadata("nothing", {"no": "op"})
diff --git a/artiq/master/worker_db.py b/artiq/master/worker_db.py
index f52162af22..3cf1732559 100644
--- a/artiq/master/worker_db.py
+++ b/artiq/master/worker_db.py
@@ -166,6 +166,8 @@ def get(self, key, archive=False):
 
     def set_metadata(self, key, metadata):
         if key:
+            if key not in self.local:
+                logger.warning(f"Key '{key}' not found in dataset.")
             self.hdf5_attributes["datasets/" + key] = metadata
         else:
             self.hdf5_attributes["datasets"] = metadata