diff --git a/include/fmt/std.h b/include/fmt/std.h
index 2eb7f64feb19c..a2a4806f234b9 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -409,6 +409,9 @@ template <typename T> struct is_bit_reference_like {
 
 }  // namespace detail
 
+// We can't use std::vector<bool, Allocator>::reference and
+// std::bitset<N>::reference because the compiler can't deduce Allocator and N
+// in partial specialization.
 FMT_EXPORT
 template <typename BitRef, typename Char>
 struct formatter<BitRef, Char,