diff --git a/boxedcpp/proxy_functions.hpp b/boxedcpp/proxy_functions.hpp
index cb2d6cf..4758975 100644
--- a/boxedcpp/proxy_functions.hpp
+++ b/boxedcpp/proxy_functions.hpp
@@ -107,8 +107,8 @@ class Proxy_Function
 class Dynamic_Proxy_Function : public Proxy_Function
 {
   public:
-    Dynamic_Proxy_Function(const boost::function<Boxed_Value (const std::vector<Boxed_Value> &)> &t_f)
-      : m_f(t_f)
+    Dynamic_Proxy_Function(const boost::function<Boxed_Value (const std::vector<Boxed_Value> &)> &t_f, int arity=-1)
+      : m_f(t_f), m_arity(arity)
     {
     }
 
@@ -116,7 +116,12 @@ class Dynamic_Proxy_Function : public Proxy_Function
 
     virtual Boxed_Value operator()(const std::vector<Boxed_Value> &params)
     {
-      return m_f(params);
+      if (m_arity < 0 || params.size() == size_t(m_arity))
+      {
+        return m_f(params);
+      } else {
+        throw std::range_error("Incorrect number of parameters");
+      } 
     }
 
     virtual std::vector<Type_Info> get_param_types()
@@ -126,6 +131,7 @@ class Dynamic_Proxy_Function : public Proxy_Function
 
   private:
     boost::function<Boxed_Value (const std::vector<Boxed_Value> &)> m_f;
+    int m_arity;
 };
 
 template<typename Func>
@@ -165,6 +171,8 @@ Boxed_Value dispatch(const std::vector<std::pair<const std::string, boost::share
       return (*itr->second)(plist);
     } catch (const std::bad_cast &) {
       //try again
+    } catch (const std::range_error &) {
+      //invalid num params, try again
     }
   }
 
diff --git a/wesley/main.cpp b/wesley/main.cpp
index 26bb891..58bcd71 100644
--- a/wesley/main.cpp
+++ b/wesley/main.cpp
@@ -82,16 +82,6 @@ std::string concat_string(const std::string &s1, const std::string &s2) {
 }
 
 const Boxed_Value add_two(BoxedCPP_System &ss, const std::vector<Boxed_Value> &vals) {
-  //We can check the arity if we want, or in this case just allow the dispatch
-  //to do it for us:
-  if (vals.size() != 2)
-  {
-    //I very much need to do a cleanup and provide a custom set of
-    //bad_cast and wrong_arity exceptions so they can be caught more
-    //cleanly
-    throw std::runtime_error("Wrong number of args");
-  }
-
   return dispatch(ss.get_function("+"), vals);
 }
 
@@ -414,7 +404,7 @@ BoxedCPP_System build_eval_system() {
     ss.register_function(boost::function<std::string (const std::string &, const std::string &)>(concat_string), "concat_string");
 
   ss.register_function(boost::shared_ptr<Proxy_Function>(
-        new Dynamic_Proxy_Function(boost::bind(&add_two, boost::ref(ss), _1))), "add_two");
+        new Dynamic_Proxy_Function(boost::bind(&add_two, boost::ref(ss), _1), 2)), "add_two");
 
 
     return ss;