diff --git a/util/streamlayerformat.py b/util/streamlayerformat.py
index 72c350a37..fde9d04ad 100644
--- a/util/streamlayerformat.py
+++ b/util/streamlayerformat.py
@@ -20,7 +20,7 @@ class StreamLayerMerger(TarLayerFormat):
     self.prefix_trie = marisa_trie.Trie()
     self.prefix_encountered = []
 
-  def after_tar_layer(stream, current_layer):
+  def after_tar_layer(self, current_layer):
       # Update the tries.
       self.path_trie = marisa_trie.Trie(self.path_encountered)
       self.prefix_trie = marisa_trie.Trie(self.prefix_encountered)