diff --git a/fmt/format.h b/fmt/format.h
index d5249326..27137d4d 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -1031,6 +1031,8 @@ struct Value {
 // allow storage in internal::MemoryBuffer.
 struct format_arg : internal::Value {
   Type type;
+
+  explicit operator bool() const noexcept { return type != NONE; }
 };
 
 namespace internal {