diff --git a/lib/oauth/request_proxy/base.rb b/lib/oauth/request_proxy/base.rb
index 4ffd8487..c1a8c57a 100644
--- a/lib/oauth/request_proxy/base.rb
+++ b/lib/oauth/request_proxy/base.rb
@@ -76,7 +76,7 @@ def parameters
     end
 
     def parameters_for_signature
-      parameters.reject { |k,v| k == "oauth_signature" || unsigned_parameters.include?(k)}
+      parameters.select { |k,v| not signature_and_unsigned_parameters.include?(k) }
     end
 
     def oauth_parameters
@@ -87,6 +87,10 @@ def non_oauth_parameters
       parameters.reject { |k,v| OAuth::PARAMETERS.include?(k) }
     end
 
+    def signature_and_unsigned_parameters
+      unsigned_parameters+["oauth_signature"]
+    end
+
     # See 9.1.2 in specs
     def normalized_uri
       u = URI.parse(uri)