Being able to tackle 'BlankObject instances' when extending:

A BlankObject class is considered not to have meta_class and unextend defined, but does respond to extend (e.g. Liquid::Strainer)
See also: http://rubydoc.info/gems/liquid/2.3.0/Liquid/Strainer

lib/unextendable/object.rb

@@ -12,20 +12,24 @@ def method_procs
 
   def extend(*modules)
     modules.each do |mod|
-      wrap_unextendable_module mod if mod.unextendable?
-      add_extended_module mod
+      if respond_to?(:meta_class)
+        wrap_unextendable_module mod if mod.unextendable?
+        add_extended_module mod
+      end
       super(mod)
     end
   end
 
   def unextend(*modules, &block)
-    if modules.empty?
-      meta_class.extended_modules.delete_if do |mod|
-        unextend? mod, &block
-      end
-    else
-      modules.each do |mod|
-        meta_class.extended_modules.delete mod if unextend? mod, &block
+    if respond_to?(:meta_class)
+      if modules.empty?
+        meta_class.extended_modules.delete_if do |mod|
+          unextend? mod, &block
+        end
+      else
+        modules.each do |mod|
+          meta_class.extended_modules.delete mod if unextend? mod, &block
+        end
       end
     end
   end

test/blank_object_test.rb

@@ -0,0 +1,52 @@
+require File.expand_path("../test_helper", __FILE__)
+
+class BlankObjectTest < Test::Unit::TestCase
+
+  # ==================================================================================================================================
+  # A BlankObject class is considered not to have meta_class and unextend defined, but does respond to extend (e.g. Liquid::Strainer)
+  # See also: http://rubydoc.info/gems/liquid/2.3.0/Liquid/Strainer
+  # ==================================================================================================================================
+
+  context "A blank object instance" do
+    setup do
+      module Hi
+        unextendable
+        def say_hi
+          "Hi!"
+        end
+      end
+      class BlankObject
+        def meta_class
+          raise NoMethodError
+        end
+        def unextend
+          raise NoMethodError
+        end
+        def respond_to?(symbol, include_private = false)
+          return false if %w(meta_class unextend).include? symbol.to_s
+          super
+        end
+      end
+    end
+
+    should "have extend defined" do
+      assert BlankObject.new.extend(Hi)
+    end
+
+    should "not have meta_class and unextend defined" do
+      assert_raises NoMethodError do
+        BlankObject.new.meta_class
+      end
+      assert_raises NoMethodError do
+        BlankObject.new.unextend
+      end
+    end
+
+    should "be extendable" do
+      b = BlankObject.new
+      b.extend Hi
+      assert_equal "Hi!", b.say_hi
+    end
+  end
+
+end