diff --git a/include/fmt/format.h b/include/fmt/format.h
index 9b5f4365..9f1178bd 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -550,7 +550,7 @@ FMT_CONSTEXPR inline size_t compute_width(string_view s) {
     size_t* count;
     FMT_CONSTEXPR void operator()(uint32_t cp, int error) const {
       *count +=
-          size_t(1 +
+          detail::to_unsigned(1 +
           (error == 0 && cp >= 0x1100 &&
            (cp <= 0x115f ||  // Hangul Jamo init. consonants
             cp == 0x2329 ||  // LEFT-POINTING ANGLE BRACKET〈