diff --git a/src/calibre/gui2/viewer/web_view.py b/src/calibre/gui2/viewer/web_view.py
index febf98dd3d..c54e7cd376 100644
--- a/src/calibre/gui2/viewer/web_view.py
+++ b/src/calibre/gui2/viewer/web_view.py
@@ -167,7 +167,7 @@ class UrlSchemeHandler(QWebEngineUrlSchemeHandler):
                     import json
                     from calibre.srv.books import get_mathjax_manifest
                     self.mathjax_manifest = json.dumps(get_mathjax_manifest()['files'])
-                    send_reply(rq, 'application/json', self.mathjax_manifest)
+                    send_reply(rq, 'application/json', as_bytes(self.mathjax_manifest))
                     return
             path = os.path.abspath(os.path.join(self.mathjax_dir, '..', name))
             if path.startswith(self.mathjax_dir):