Object has type qualifiers that prevent match (function overload not found)

I have a simple class, intended to convert integers to byte arrays.

class mc_int {
    private: 
        int val;      //actual int
    public: 
        int value();  //Returns value
        int value(int);  //Changes and returns value
        mc_int();  //Default constructor
        mc_int(int);//Create from int
        void asBytes(char*); //generate byte array

        mc_int& operator=(int);
        mc_int& operator=(const mc_int&);

        bool endianity;  //true for little
};

For conversion and simpler usage, I decided to add operator= methods. But I think my implementation of mc_int& operator=(const mc_int&); is incorrect.

mc_int& mc_int::operator=(const mc_int& other) {
            val = other.value();  
            //    |-------->Error: No instance of overloaded function matches the argument list and object (object has type quelifiers that prevent the match)
}

What could this be? I tried to use other->value(), but that was wrong too.