overloaded operator deduction using user-defined conversions in c++11

My problem may be related to http://ift.tt/1LPRtHz. But in my case I have const complex types:

class Complex {
public:
   int data;

   Complex(int i) : data(i) {}
   bool operator < (const Complex& other) const { return data < other.data; }
};
class Holder {
public:
    Complex data;

    Holder(int i) : data(i) {}
    operator const Complex&() const { return data; }
};
//...
Holder a(1), b(2);
assert(a < b); //Error 

Compiler error in g++ 4.9: no match for ‘operator<’ (operand types are ‘Holder‘ and ‘Holder‘)

Any idear how to fix this?

Btw. I need that conversion to only allow casts to const types.