Error: passing x as 'this' argument of x discards qualifiers

I'm writing a HeartbeatManager for my application (in visual studio). The data of 1 heartbeat is stored in a Heart object. The Heart objects are stored in an std::set. For this reason, I am implementing the operator=, operator< and operator> overloads.

When defining these functions you can only use const members. I tried to do this but still get an error saying I'm not:

passing const HeartbeatManager::Heart' as 'this' argument of 'bool HeartbeatManager::Heart::operator<(const HeartbeatManager::Heart&)' 
discards qualifiers [-fpermissive]

Here is the code. I don't see where I'm using a non-constant:

class HeartbeatManager
{
public:
    class Heart
    {
    public:
        Heart(const IPAddress _ip, const uint16_t _port, int _lifetime = 5000)
            : ip(_ip), port(_port), lifetime(_lifetime) {}

        const IPAddress ip;
        const uint16_t port;
        int lifetime;
        /**
        * Ages the Heart, returns whether it survived (lifetime after aging > 0)
        */
        bool age(int ms) 
        {
            this->lifetime -= ms;
            return 0 < this->lifetime;
        }

        // overloaded operators so heart struct can be sorted and used in map
        bool operator=(const Heart& o) {
            return ip == o.ip && port == o.port;
        }

        bool operator<(const Heart& o) {
            return (uint32_t)ip < (uint32_t)o.ip || (ip == o.ip && port < o.port);
        }

        bool operator>(const Heart& o) {
            return (uint32_t)ip > (uint32_t)o.ip || (ip == o.ip && port > o.port);
        }
    };
    void heartbeat(IPAddress ip, uint16_t port, int sec = 5000);
};

I'm very new to C++. So if this is a bad practice for making an object fit the set requirements feel free to let me know.