diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -215,6 +215,14 @@
     return int(hex_digit, 16)
 
 
+class Block(dict):
+    """represent a block of the Trie
+
+    contains up to 16 entry indexed from 0 to 15"""
+
+    pass
+
+
 def _build_trie(index):
     """build a nodemap trie
 
@@ -223,7 +231,7 @@
     Each block is a dictionnary with key in `[0, 15]`. Value are either
     another block or a revision number.
     """
-    root = {}
+    root = Block()
     for rev in range(len(index)):
         hex = nodemod.hex(index[rev][7])
         _insert_into_block(index, 0, root, rev, hex)
@@ -252,7 +260,7 @@
         other_hex = nodemod.hex(index[entry][7])
         other_rev = entry
         while current_hex[level] == other_hex[level]:
-            new = {}
+            new = Block()
             block[_to_int(current_hex[level])] = new
             block = new
             level += 1