diff --git a/lib/phoenix/socket/serializers/v1_json_serializer.ex b/lib/phoenix/socket/serializers/v1_json_serializer.ex
index 174f70c351..cd96fe8cc9 100644
--- a/lib/phoenix/socket/serializers/v1_json_serializer.ex
+++ b/lib/phoenix/socket/serializers/v1_json_serializer.ex
@@ -28,9 +28,15 @@ defmodule Phoenix.Socket.V1.JSONSerializer do
 
   @impl true
   def decode!(message, _opts) do
-    message
-    |> Phoenix.json_library().decode!()
-    |> Phoenix.Socket.Message.from_map!()
+    payload = Phoenix.json_library().decode!(message)
+
+    case payload do
+      %{} ->
+        Phoenix.Socket.Message.from_map!(payload)
+
+      other ->
+        raise "V1 JSON Serializer expected a map, got #{inspect(other)}"
+    end
   end
 
   defp encode_v1_fields_only(%Message{} = msg) do
diff --git a/test/phoenix/socket/v1_json_serializer_test.exs b/test/phoenix/socket/v1_json_serializer_test.exs
index 9e25aabaf9..7f0cfc9eb7 100644
--- a/test/phoenix/socket/v1_json_serializer_test.exs
+++ b/test/phoenix/socket/v1_json_serializer_test.exs
@@ -6,6 +6,7 @@ defmodule Phoenix.Socket.V1.JSONSerializerTest do
   # v1 responses must not contain join_ref
   @serializer V1.JSONSerializer
   @v1_msg_json "{\"event\":\"e\",\"payload\":\"m\",\"ref\":null,\"topic\":\"t\"}"
+  @v1_bad_json "[null,null,\"t\",\"e\",{\"m\":1}]"
   @v1_reply_json "{\"event\":\"phx_reply\",\"payload\":{\"response\":null,\"status\":null},\"ref\":\"null\",\"topic\":\"t\"}"
   @v1_fastlane_json "{\"event\":\"e\",\"payload\":\"m\",\"ref\":null,\"topic\":\"t\"}"
 
@@ -33,7 +34,15 @@ defmodule Phoenix.Socket.V1.JSONSerializerTest do
 
   test "decode!/2 decodes `Phoenix.Socket.Message` from JSON" do
     assert %Message{topic: "t", event: "e", payload: "m"} ==
-      decode!(@serializer, @v1_msg_json, opcode: :text)
+             decode!(@serializer, @v1_msg_json, opcode: :text)
+  end
+
+  test "decode!/2 raise a PayloadFormatException if the JSON doesn't contain a map" do
+    assert_raise(
+      RuntimeError,
+      "V1 JSON Serializer expected a map, got [nil, nil, \"t\", \"e\", %{\"m\" => 1}]",
+      fn -> decode!(@serializer, @v1_bad_json, opcode: :text) end
+    )
   end
 
   test "fastlane!/1 encodes a broadcast into a message as JSON" do