diff --git a/fmt/ostream.h b/fmt/ostream.h
index 0234a43e..7e13a5a7 100644
--- a/fmt/ostream.h
+++ b/fmt/ostream.h
@@ -90,8 +90,8 @@ public:
 }  // namespace internal
 
 // Formats a value.
-template <typename Char, typename ArgFormatter, typename T>
-void format_arg(BasicFormatter<Char, ArgFormatter> &f,
+template <typename Char, typename ArgFormatter_, typename T>
+void format_arg(BasicFormatter<Char, ArgFormatter_> &f,
                 const Char *&format_str, const T &value) {
   internal::MemoryBuffer<Char, internal::INLINE_BUFFER_SIZE> buffer;