diff --git a/include/fmt/format.h b/include/fmt/format.h
index f63aefc91823..46522b6a8708 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -122,7 +122,7 @@ namespace std {
 template <class T> struct iterator_traits<fmt::basic_appender<T>> {
   using iterator_category = output_iterator_tag;
   using value_type = T;
-  using difference_type = void;
+  using difference_type = decltype(static_cast<int*>(nullptr) - static_cast<int*>(nullptr));
   using pointer = void;
   using reference = void;
 };