diff --git a/fmt/format.h b/fmt/format.h
index c2a4275a..68892d80 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -2157,10 +2157,10 @@ inline uint64_t make_type(const T &arg) {
   return MakeValue< BasicFormatter<char> >::type(arg);
 }
 
-template <unsigned N, bool/*IsPacked*/= (N < ArgList::MAX_PACKED_ARGS)>
+template <std::size_t N, bool/*IsPacked*/= (N < ArgList::MAX_PACKED_ARGS)>
 struct ArgArray;
 
-template <unsigned N>
+template <std::size_t N>
 struct ArgArray<N, true/*IsPacked*/> {
   typedef Value Type[N > 0 ? N : 1];
 
@@ -2178,7 +2178,7 @@ struct ArgArray<N, true/*IsPacked*/> {
   }
 };
 
-template <unsigned N>
+template <std::size_t N>
 struct ArgArray<N, false/*IsPacked*/> {
   typedef Arg Type[N + 1]; // +1 for the list end Arg::NONE