diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 1b38913359d..c3327e1d384 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4051,7 +4051,7 @@ namespace __format
       } _M_pc;
 
       constexpr explicit
-      _Scanner(basic_string_view<_CharT> __str, size_t __nargs = -1)
+      _Scanner(basic_string_view<_CharT> __str, size_t __nargs = (size_t)-1)
       : _M_pc(__str, __nargs)
       { }