diff --git a/src/serialization/json_archive.h b/src/serialization/json_archive.h
index f906b5d3b..04436c21c 100644
--- a/src/serialization/json_archive.h
+++ b/src/serialization/json_archive.h
@@ -113,7 +113,7 @@ struct json_archive;
 template <>
 struct json_archive<true> : public json_archive_base<std::ostream, true>
 {
-  json_archive(stream_type &s, bool indent = false) : base_type(s, indent) { }
+  json_archive(stream_type &s, bool indent = false) : base_type(s, indent), inner_array_size_(0) { }
 
   template<typename T>
   static auto promote_to_printable_integer_type(T v) -> decltype(+v)