diff --git a/http2/adapter/nghttp2_callbacks.cc b/http2/adapter/nghttp2_callbacks.cc
index 648a521..c3abeef 100644
--- a/http2/adapter/nghttp2_callbacks.cc
+++ b/http2/adapter/nghttp2_callbacks.cc
@@ -178,6 +178,9 @@
       return NGHTTP2_ERR_CALLBACK_FAILURE;
     case Http2VisitorInterface::HEADER_RST_STREAM:
       return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
+    default:
+      // Unexpected value.
+      return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
   }
 }