diff --git a/lib/backend.py b/lib/backend.py
index e2aac3bb2084705ec4ee732dd66f56e924349adf..7a9e0d8419cf88b1d7ee606ad6a5d00af1815d35 100644
--- a/lib/backend.py
+++ b/lib/backend.py
@@ -78,6 +78,7 @@ def _Decompress(data):
   @return: Decompressed data
 
   """
+  assert isinstance(data, (list, tuple))
   assert len(data) == 2
   (encoding, content) = data
   if encoding == constants.RPC_ENCODING_NONE: