diff --git a/lib/microwaveprop/application.ex b/lib/microwaveprop/application.ex
index 06a241bd..1e3f3948 100644
--- a/lib/microwaveprop/application.ex
+++ b/lib/microwaveprop/application.ex
@@ -127,8 +127,14 @@ defmodule Microwaveprop.Application do
         # Erlang's node() name in non-K8s environments.
         node_name = System.get_env("POD_IP") || to_string(node())
 
-        {Phoenix.PubSub,
-         name: Microwaveprop.PubSub, adapter: Phoenix.PubSub.Redis, redis_opts: [url: url], node_name: node_name}
+        # phoenix_pubsub_redis 3.x: `redis_opts` accepts either a URL
+        # binary OR a keyword list of host/port/password/etc. The
+        # 3.1's deprecation warning ("Move :url inside the :redis_opts
+        # option instead") is misleading — `[url: url]` gets forwarded
+        # to Redix which rejects it as an unknown option (the valid
+        # keys are :host, :port, :password, …, NOT :url). The bare URL
+        # form takes the binary-clause and works correctly.
+        {Phoenix.PubSub, name: Microwaveprop.PubSub, adapter: Phoenix.PubSub.Redis, redis_opts: url, node_name: node_name}
     end
   end