diff --git a/test/unit/test_identity_map_middleware.rb b/test/unit/test_identity_map_middleware.rb
index <HASH>..<HASH> 100644
--- a/test/unit/test_identity_map_middleware.rb
+++ b/test/unit/test_identity_map_middleware.rb
@@ -8,7 +8,7 @@ class IdentityMapMiddlewareTest < Test::Unit::TestCase
     @app ||= Rack::Builder.new do
       use MongoMapper::Middleware::IdentityMap
       map "/" do
-        run lambda {|env| [200, {}, ''] }
+        run lambda {|env| [200, {}, []] }
       end
       map "/fail" do
         run lambda {|env| raise "FAIL!" }
Fix middleware test on Ruby <I>

Rack's MockResponse calls #each on the body, which is not defined on String in Ruby <I>