diff --git a/src/allmydata/immutable/encode.py b/src/allmydata/immutable/encode.py
index 9351df501..e743ce766 100644
--- a/src/allmydata/immutable/encode.py
+++ b/src/allmydata/immutable/encode.py
@@ -711,3 +711,6 @@ class Encoder(object):
         return self.uri_extension_data
     def get_uri_extension_hash(self):
         return self.uri_extension_hash
+
+    def set_size(self, size):
+        raise NotImplementedError()