Problems with custom converter

Hi,

recently I am having very rough time with code using luabind so I
finally decided to ask here.

In my code I have a class like this, color.h:

struct Color3b
{
  const static Color3b White;
  const static Color3b Black;

  Color3b() {}

  Color3b(Byte r_, Byte g_, Byte b_) : r(r_), g(g_), b(b_) {}

  Byte r, g, b;
};

then in some cpp file:

namespace luabind
{
  template <>
  struct default_converter<Color3b> : native_converter_base<Color3b>
  {
    static int compute_score(lua_State* L, int index)
    {
      return lua_type(L, index) == LUA_TTABLE ? 0 : -1;
    }

    Color3b from(lua_State* L, int index)
    {
      Color3b ret;

     //snip

      return ret;
    }

    void to(lua_State* L, Color3b const& color)
    {
      object obj = newtable(L);

      obj[1] = color.r;
      obj[2] = color.g;
      obj[3] = color.b;

      obj.push(L);
    }
  };

  template <>
  struct default_converter<Color3b const&>
    : default_converter<Color3b>
  {};
}

then later in same file:

Color3b color3b = luabind::object_cast<Color3b>(params["normal"]);

Problem is that this cast always results with exception "unable to
make cast". Program never enters compute_score() for provided
converter, in fact break point is marked invalid because this code
seem to be unused at all. I know that params["normal'] is a valid lua
object having needed 3 values inside
(in lua it is like: color = {1123, 123, 123}).

What is the possible reason for object_cast not to see this converter?
Why is compile-time expression object_cast<Color3b> not a compile
error if no proper converter/conversion trait is found?

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