Invalid operand to binary expression on std::max_element

I'm trying to use std::max_element to find the largest element in a std::forward_list of a defined structure. Here is the code below:

//.h file:
#include <unordered_map>
#include <forward_list>

// my structure: 
struct A
{
    uint8_t length;
    bool reverseMatch;

    bool operator<(const A& rhs);
    A() : length(0), reverseMatch(false) {}
};

using Alias = std::unordered_map<uint32_t, std::forward_list<A>>;

class B
{
    Alias data;

public:
    parse(string inputFilename);
    A getBestMatch(uint32_t lineNumber);
};

Troublesome function:

//.cpp file
#include <sstream>
#include <algorithm>
#include <string>
#include "file.h"

bool A::operator<(const MUMmatch& rhs)
{
    return (this->length < rhs.length);
}
A B::getBestMatch(uint32_t lineNumber)
{
    A ret;
    auto dataIter = this->data.find(lineNumber);
    if (dataIter != data.end())
    {
        ret = *(std::max_element(dataIter->second.begin(), dataIter->second.end()));//!!!ERROR!!!
    }
    return ret;
}

The error that I'm getting is "Invalid operands to binary expression ('const A' and 'const A')". I'm not sure why my operator< overload is having issues here. Do I need to define other operands as well? If so, why? All documentation that I have read states that std::max_element uses the < operator. Thanks!