diff --git a/canonicaljson.py b/canonicaljson.py
index 8dee682..5a4c2ef 100644
--- a/canonicaljson.py
+++ b/canonicaljson.py
@@ -26,7 +26,13 @@ def _default(obj):
     if type(obj) is frozendict:
         # fishing the protected dict out of the object is a bit nasty,
         # but we don't really want the overhead of copying the dict.
-        return obj._dict
+        try:
+            return obj._dict
+        except AttributeError:
+            # When the C implementation of frozendict is used,
+            # there isn't a `_dict` attribute with a dict
+            # so we resort to making a copy of the frozendict
+            return dict(obj)
     raise TypeError(
         "Object of type %s is not JSON serializable" % obj.__class__.__name__
     )