diff --git a/sygnal/gcmpushkin.py b/sygnal/gcmpushkin.py
index f725b51f..91a81811 100644
--- a/sygnal/gcmpushkin.py
+++ b/sygnal/gcmpushkin.py
@@ -371,10 +371,13 @@ async def _dispatch_notification_unlimited(
             body["priority"] = "normal" if n.prio == "low" else "high"
 
             for retry_number in range(0, MAX_TRIES):
+                # Should only ever be one of `to` or `registration_ids` fields
                 if len(pushkeys) == 1:
                     body["to"] = pushkeys[0]
+                    body.pop("registration_ids", None)
                 else:
                     body["registration_ids"] = pushkeys
+                    body.pop("to", None)
 
                 log.info("Sending (attempt %i) => %r", retry_number, pushkeys)