Should use Char not char, as Char is typedef'd to the correct type (char or wchar_t)
diff --git a/format.cc b/format.cc
index c3170cd..090e3f4 100644
--- a/format.cc
+++ b/format.cc
@@ -633,7 +633,7 @@
       } else {
         out = writer.GrowBuffer(1);
       }
-      *out = static_cast<char>(arg.int_value);
+      *out = static_cast<Char>(arg.int_value);
       break;
     }
     case STRING: {