Fixed iteration bug over null values.
diff --git a/include/json/value.h b/include/json/value.h
index 3884b08..72fcabd 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -917,6 +917,8 @@
    private:
 #ifndef JSON_VALUE_USE_INTERNAL_MAP
       Value::ObjectValues::iterator current_;
+      // Indicates that iterator is for a null value.
+      bool isNull_;
 #else
       union
       {