diff --git a/test/format-test.cc b/test/format-test.cc
index c995ff69..8ed524cc 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -2439,6 +2439,8 @@ struct custom_char {
   operator int() const { return value; }
 };
 
+int to_ascii(custom_char c) { return c; }
+
 FMT_BEGIN_NAMESPACE
 template <> struct is_char<custom_char> : std::true_type {};
 FMT_END_NAMESPACE