diff --git a/lib/capybara/server.rb b/lib/capybara/server.rb
index 0fec181f..38707072 100644
--- a/lib/capybara/server.rb
+++ b/lib/capybara/server.rb
@@ -12,7 +12,7 @@ module Capybara
 
       def call(env)
         if env["PATH_INFO"] == "/__identify__"
-          [200, {}, @app.object_id.to_s]
+          [200, {}, [@app.object_id.to_s]]
         else
           @app.call(env)
         end