diff --git a/lib/poundpay/resource.rb b/lib/poundpay/resource.rb
index f525efe..a1168d3 100644
--- a/lib/poundpay/resource.rb
+++ b/lib/poundpay/resource.rb
@@ -7,7 +7,9 @@ class Resource < ActiveResource::Base
     self.format = Formats::UrlencodedJsonFormat
 
     class << self
-      attr_accessor_with_default(:primary_key, 'sid')
+      def primary_key
+        'sid'
+      end
 
       # Modified default to not use an extension
       def element_path(id, prefix_options = {}, query_options = nil)
diff --git a/poundpay.gemspec b/poundpay.gemspec
index 34f458c..a48680c 100644
--- a/poundpay.gemspec
+++ b/poundpay.gemspec
@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
 
   s.rubyforge_project = "poundpay"
 
-  s.add_dependency("activeresource", "~> 3.0.0")
+  s.add_dependency("activeresource", "~> 3.1.0")
 
   s.add_development_dependency("rspec", ">= 2.0")
   s.add_development_dependency("wirble")