diff --git a/contrib/pyln-proto/pyln/proto/message/message.py b/contrib/pyln-proto/pyln/proto/message/message.py
index 33b879c9b76e..31a1511bafe2 100644
--- a/contrib/pyln-proto/pyln/proto/message/message.py
+++ b/contrib/pyln-proto/pyln/proto/message/message.py
@@ -600,7 +600,8 @@ def set_field(self, field: str, val: Any) -> None:
         f = self.messagetype.find_field(field)
         if f is None:
             raise ValueError("Unknown field {}".format(field))
-
+        if isinstance(f.fieldtype, LengthFieldType):
+            raise ValueError("Cannot specify implied length field {}".format(field))
         if isinstance(val, str):
             val, remainder = f.fieldtype.val_from_str(val)
             if remainder != '':