diff --git a/draft-irtf-cfrg-vdaf.md b/draft-irtf-cfrg-vdaf.md
index 5bb8fab6..15c9efa8 100644
--- a/draft-irtf-cfrg-vdaf.md
+++ b/draft-irtf-cfrg-vdaf.md
@@ -3991,8 +3991,7 @@ def is_valid(Poplar1, agg_param, previous_agg_params):
 
     (level, prefixes) = agg_param
     (last_level, last_prefixes) = previous_agg_params[-1]
-    # The empty prefix 0 is always there.
-    last_prefixes_set = set(list(last_prefixes) + [0])
+    last_prefixes_set = set(last_prefixes)
 
     # Check that level increased.
     if level <= last_level