diff --git a/include/fmt/format.h b/include/fmt/format.h
index 7c607dbd..15f2ae92 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -1107,7 +1107,7 @@ FMT_CONSTEXPR auto count_digits(UInt n) -> int {
     return (FMT_BUILTIN_CLZ(static_cast<uint32_t>(n) | 1) ^ 31) / BITS + 1;
 #endif
   // Lambda avoids unreachable code warnings from NVHPC.
-  return [](UInt m) {
+  return [](UInt m) -> int {
     int num_digits = 0;
     do {
       ++num_digits;