diff --git a/piker/data/types.py b/piker/data/types.py
index 1359526c..c37de233 100644
--- a/piker/data/types.py
+++ b/piker/data/types.py
@@ -80,6 +80,8 @@ class Struct(
             msgspec.msgpack.Encoder().encode(self)
         )
 
+    # NOTE XXX: this won't work on frozen types!
+    # use ``.copy()`` above in such cases.
     def typecast(
         self,
         # fields: Optional[list[str]] = None,