diff --git a/lib/phoenix_live_view.ex b/lib/phoenix_live_view.ex
index 0d8578a469..ff77ea9e74 100644
--- a/lib/phoenix_live_view.ex
+++ b/lib/phoenix_live_view.ex
@@ -2245,8 +2245,8 @@ defmodule Phoenix.LiveView do
   an `Phoenix.LiveView.AsyncResult` struct holding the status of the operation
   and the result when the function completes.
 
-  The function must return either a map or a keyword list with the assigns
-  to merge into the socket.
+  The function must return either `{:ok, assigns}` or `{:error, reason}`,
+  where `assigns` is a map with the keys passed to `assign_async/3`.
 
   The task is only started when the socket is connected.
 
diff --git a/lib/phoenix_live_view/async.ex b/lib/phoenix_live_view/async.ex
index 64653c342a..ff4c107eff 100644
--- a/lib/phoenix_live_view/async.ex
+++ b/lib/phoenix_live_view/async.ex
@@ -363,7 +363,7 @@ defmodule Phoenix.LiveView.Async do
 
   defp handle_kind(socket, _maybe_component, :assign, keys, result) do
     case result do
-      {:ok, {:ok, assigns}} when is_map(assigns) or is_list(assigns) ->
+      {:ok, {:ok, assigns}} when is_map(assigns) ->
         new_assigns =
           for {key, val} <- assigns do
             {key, AsyncResult.ok(get_current_async!(socket, key), val)}
diff --git a/test/phoenix_live_view/integrations/assign_async_test.exs b/test/phoenix_live_view/integrations/assign_async_test.exs
index 7e16d89e03..226346b786 100644
--- a/test/phoenix_live_view/integrations/assign_async_test.exs
+++ b/test/phoenix_live_view/integrations/assign_async_test.exs
@@ -32,6 +32,15 @@ defmodule Phoenix.LiveView.AssignAsyncTest do
       assert render(lv)
     end
 
+    test "keyword list return", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, "/assign_async?test=bad_keyword")
+
+      assert render_async(lv) =~
+               "expected assign_async to return {:ok, map} of\\nassigns for [:data] or {:error, reason}, got: {:ok, [data: 123]}"
+
+      assert render(lv)
+    end
+
     test "valid return", %{conn: conn} do
       {:ok, lv, _html} = live(conn, "/assign_async?test=ok")
       assert render_async(lv) =~ "data: 123"
@@ -222,7 +231,9 @@ defmodule Phoenix.LiveView.AssignAsyncTest do
 
     test "valid return", %{conn: conn} do
       {:ok, lv, _html} = live(conn, "/assign_async?test=sup_ok")
-      assert render_async(lv) =~ "data: 123"
+      html = render_async(lv)
+      assert html =~ "data: 123"
+      refute html =~ "expected assign_async to return"
     end
 
     test "raise during execution", %{conn: conn} do
diff --git a/test/support/live_views/assign_async.ex b/test/support/live_views/assign_async.ex
index 95153b654c..69e55ce476 100644
--- a/test/support/live_views/assign_async.ex
+++ b/test/support/live_views/assign_async.ex
@@ -40,13 +40,17 @@ defmodule Phoenix.LiveViewTest.Support.AssignAsyncLive do
     {:ok, assign_async(socket, :data, fn -> {:ok, %{bad: 123}} end)}
   end
 
+  def mount(%{"test" => "bad_keyword"}, _session, socket) do
+    {:ok, assign_async(socket, :data, fn -> {:ok, data: 123} end)}
+  end
+
   def mount(%{"test" => "ok"}, _session, socket) do
     {:ok, assign_async(socket, :data, fn -> {:ok, %{data: 123}} end)}
   end
 
   def mount(%{"test" => "sup_ok"}, _session, socket) do
     {:ok,
-     assign_async(socket, :data, fn -> {:ok, data: 123} end, supervisor: TestAsyncSupervisor)}
+     assign_async(socket, :data, fn -> {:ok, %{data: 123}} end, supervisor: TestAsyncSupervisor)}
   end
 
   def mount(%{"test" => "raise"}, _session, socket) do