diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 26bb9152a..17898bc69 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -18,6 +18,8 @@ jobs:
         run: mix deps.get
       - name: Check formatting
         run: mix format --check-formatted
+      - name: Check warnings
+        run: mix compile --warnings-as-errors
       - name: Run tests
         run: elixir --cookie "COOKIEFORTESTS" -S mix test
       - name: Install Node
diff --git a/lib/livebook/completion.ex b/lib/livebook/completion.ex
index b01db3129..8b1c27c89 100644
--- a/lib/livebook/completion.ex
+++ b/lib/livebook/completion.ex
@@ -676,7 +676,15 @@ defmodule Livebook.Completion do
   def cursor_context(charlist, opts) when is_list(charlist) and is_list(opts) do
     chunked = Enum.chunk_by(charlist, &(&1 == ?\n))
 
-    case List.last(chunked, []) do
+    # TODO: Use List.last/2 on Elixir v1.12+
+    last =
+      if chunked == [] do
+        []
+      else
+        List.last(chunked)
+      end
+
+    case last do
       [?\n | _] -> do_cursor_context([], opts)
       rest -> do_cursor_context(rest, opts)
     end