diff --git a/src/app_helper.cc b/src/app_helper.cc
index 11f7ca5e..40486057 100644
--- a/src/app_helper.cc
+++ b/src/app_helper.cc
@@ -146,7 +146,7 @@ void print_timer()
 namespace {
 void print_frame_hd(const nghttp2_frame_hd& hd)
 {
-  printf("<length=%d, flags=%u, stream_id=%d>\n",
+  printf("<length=%d, flags=0x%02x, stream_id=%d>\n",
          hd.length, hd.flags, hd.stream_id);
 }
 } // namespace