diff --git a/http2/adapter/http2_visitor_interface.h b/http2/adapter/http2_visitor_interface.h
index 8297334..4d12e42 100644
--- a/http2/adapter/http2_visitor_interface.h
+++ b/http2/adapter/http2_visitor_interface.h
@@ -52,8 +52,10 @@
   Http2VisitorInterface& operator=(const Http2VisitorInterface&) = delete;
   virtual ~Http2VisitorInterface() = default;
 
-  static constexpr int64_t kSendBlocked = 0;
-  static constexpr int64_t kSendError = -1;
+  enum : int64_t {
+    kSendBlocked = 0,
+    kSendError = -1,
+  };
   // Called when there are serialized frames to send. Should return how many
   // bytes were actually sent. May return kSendBlocked or kSendError.
   virtual int64_t OnReadyToSend(absl::string_view serialized) = 0;