diff --git a/include/fmt/format.h b/include/fmt/format.h
index af57862b..38626caf 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -235,7 +235,7 @@ inline Dest bit_cast(const Source& source) {
 }
 
 inline bool is_big_endian() {
-  auto u = 1u;
+  const auto u = 1u;
   struct bytes {
     char data[sizeof(u)];
   };