diff --git a/nbviewer/app.py b/nbviewer/app.py
index 7e980e59..5e8b05a4 100644
--- a/nbviewer/app.py
+++ b/nbviewer/app.py
@@ -460,8 +460,10 @@ def default_endpoint(self):
             default_host, default_port = url.hostname, url.port
 
             if default_port < 1 or default_port > 65535:
-                self.log.error("Received invalid port number %d through JUPYTERHUB_SERVICE_URL. "
-                               "Defaulting to 5000 instead.")
+                self.log.error(
+                    "Received invalid port number %d through JUPYTERHUB_SERVICE_URL. "
+                    "Defaulting to 5000 instead."
+                )
                 default_port = 5000
         else:
             default_host, default_port = "0.0.0.0", 5000