diff --git a/examples/guide_parallel_filling.cpp b/examples/guide_parallel_filling.cpp
index 2f0a46a3..475eb770 100644
--- a/examples/guide_parallel_filling.cpp
+++ b/examples/guide_parallel_filling.cpp
@@ -33,6 +33,8 @@ class copyable_atomic : public std::atomic<T> {
 public:
   using std::atomic<T>::atomic;
 
+  copyable_atomic() = default;
+
   // this is potentially not thread-safe
   copyable_atomic(const copyable_atomic& rhs) { this->operator=(rhs); }
 
diff --git a/include/boost/histogram/storage_adaptor.hpp b/include/boost/histogram/storage_adaptor.hpp
index 88376591..84ad22cd 100644
--- a/include/boost/histogram/storage_adaptor.hpp
+++ b/include/boost/histogram/storage_adaptor.hpp
@@ -176,9 +176,10 @@ struct map_augmentation : T {
 
   value_type& operator[](std::size_t i) { return T::operator[](i); }
 
-  value_type operator[](std::size_t i) const {
+  const value_type& operator[](std::size_t i) const {
     auto it = this->find(i);
-    return it == this->end() ? value_type() : it->second;
+    static auto null = value_type();
+    return it == this->end() ? null : it->second;
   }
 
   template <typename U>