diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index c3c52bf592b72..f99461fadc78b 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -1396,7 +1396,8 @@ inline bool isfinite(T) {
 // Converts value to Int and checks that it's in the range [0, upper).
 template <typename T, typename Int, FMT_ENABLE_IF(std::is_integral<T>::value)>
 inline Int to_nonnegative_int(T value, Int upper) {
-  FMT_ASSERT(value >= 0 && to_unsigned(value) <= to_unsigned(upper),
+  FMT_ASSERT(std::is_unsigned<Int>::value ||
+             (value >= 0 && to_unsigned(value) <= to_unsigned(upper)),
              "invalid value");
   (void)upper;
   return static_cast<Int>(value);
diff --git a/include/fmt/core.h b/include/fmt/core.h
index 0e7843b85fa8d..4bfb742fcda9c 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -402,7 +402,7 @@ template <typename T> auto convert_for_visit(T) -> monostate { return {}; }
 template <typename Int>
 FMT_CONSTEXPR auto to_unsigned(Int value) ->
     typename std::make_unsigned<Int>::type {
-  FMT_ASSERT(value >= 0, "negative value");
+  FMT_ASSERT(std::is_unsigned<Int>::value || value >= 0, "negative value");
   return static_cast<typename std::make_unsigned<Int>::type>(value);
 }