diff --git a/include/fast_float/ascii_number.h b/include/fast_float/ascii_number.h
index 69366fe..995fc47 100644
--- a/include/fast_float/ascii_number.h
+++ b/include/fast_float/ascii_number.h
@@ -31,7 +31,9 @@ fastfloat_really_inline constexpr bool has_simd_opt() noexcept {
 // able to optimize it well.
 template <typename UC>
 fastfloat_really_inline constexpr bool is_integer(UC c) noexcept {
-  return static_cast<uint8_t>(c - UC('0')) <= 9;
+  const auto d = c - UC('0');
+  // UC may be signed.
+  return d >= 0 && d <= 9;
 }
 
 #if FASTFLOAT_IS_BIG_ENDIAN