diff --git a/html5lib/html5parser.py b/html5lib/html5parser.py
index df2a6cf7..daee854c 100644
--- a/html5lib/html5parser.py
+++ b/html5lib/html5parser.py
@@ -893,7 +893,7 @@ def __init__(self, parser, tree):
                 ("body", self.startTagBody),
                 ("frameset", self.startTagFrameset),
                 (("address", "article", "aside", "blockquote", "center", "details",
-                  "details", "dir", "div", "dl", "fieldset", "figcaption", "figure",
+                  "dir", "div", "dl", "fieldset", "figcaption", "figure",
                   "footer", "header", "hgroup", "main", "menu", "nav", "ol", "p",
                   "section", "summary", "ul"),
                  self.startTagCloseP),
diff --git a/html5lib/utils.py b/html5lib/utils.py
index ea65ab6b..03f0dab7 100644
--- a/html5lib/utils.py
+++ b/html5lib/utils.py
@@ -61,6 +61,7 @@ def __init__(self, items=()):
             else:
                 _dictEntries.append((name, value))
         dict.__init__(self, _dictEntries)
+        assert len(self) == len(_dictEntries)
         self.default = None
 
     def __getitem__(self, key):