diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index 625b1b70af..424965368e 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -81,7 +81,7 @@ class DeviceMessageHandler(object):
 
     @defer.inlineCallbacks
     def send_device_message(self, sender_user_id, message_type, messages):
-        opentracing.set_tag("number of messages", len(messages))
+        opentracing.set_tag("number_of_messages", len(messages))
         opentracing.set_tag("sender", sender_user_id)
         local_messages = {}
         remote_messages = {}