diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index f45abcc9..07eee480 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -3951,9 +3951,13 @@ static int
 session_on_connection_window_update_received(nghttp2_session *session,
                                              nghttp2_frame *frame) {
   /* Handle connection-level flow control */
-  if (frame->window_update.window_size_increment == 0 ||
-      NGHTTP2_MAX_WINDOW_SIZE - frame->window_update.window_size_increment <
-          session->remote_window_size) {
+  if (frame->window_update.window_size_increment == 0) {
+    return session_handle_invalid_connection(session, frame,
+                                             NGHTTP2_PROTOCOL_ERROR, NULL);
+  }
+
+  if (NGHTTP2_MAX_WINDOW_SIZE - frame->window_update.window_size_increment <
+      session->remote_window_size) {
     return session_handle_invalid_connection(session, frame,
                                              NGHTTP2_FLOW_CONTROL_ERROR, NULL);
   }
@@ -3980,9 +3984,12 @@ static int session_on_stream_window_update_received(nghttp2_session *session,
         session, frame, NGHTTP2_PROTOCOL_ERROR,
         "WINDOW_UPADATE to reserved stream");
   }
-  if (frame->window_update.window_size_increment == 0 ||
-      NGHTTP2_MAX_WINDOW_SIZE - frame->window_update.window_size_increment <
-          stream->remote_window_size) {
+  if (frame->window_update.window_size_increment == 0) {
+    return session_handle_invalid_stream(session, frame,
+                                         NGHTTP2_PROTOCOL_ERROR);
+  }
+  if (NGHTTP2_MAX_WINDOW_SIZE - frame->window_update.window_size_increment <
+      stream->remote_window_size) {
     return session_handle_invalid_stream(session, frame,
                                          NGHTTP2_FLOW_CONTROL_ERROR);
   }