Strange behaviour of operator [] in stl::map

i was solving a program to prevent duplicate values on map, i have written the following program

    /*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

/* 
 * File:   main.cpp
 */
#include <map>
#include <limits>
#include<iostream>
using namespace std;



template<typename K, typename V>

class interval_map {
    std::map<K,V> m_map;

public:
    // constructor associates whole range of K with val by inserting (K_min, val)
    // into the map
    interval_map( V const& val) {
        m_map.insert(m_map.end(),std::make_pair(std::numeric_limits<K>::lowest(),val));
    }

    void assign( K const& keyBegin, K const& keyEnd, V const& val ) {
        if (!(keyBegin < keyEnd)) {
            return;
        }
        else {
            if (m_map.rbegin() != m_map.rend()) { 
                //get the previous key value
                auto prev_value = m_map.rbegin()->second;
                //compare with current value
                if (prev_value == val) {
                    cout << "duplicate";
                    //duplicate entry values are restricted, do nothing     
                }
                else {
                    for (auto i = keyBegin; i<keyEnd; i++) {
                        cout << i << endl;
                        m_map[i] = val;
                    }
                }

            }
        }
    }

    // look-up of the value associated with key
    V const& operator[]( K const& key ) const {
        return ( --m_map.upper_bound(key) )->second;
    }
};

int main(int argc, char** argv) {
    interval_map<unsigned int,char> test('m');
    test.assign(2,4,'k');
    test.assign(4,7, 'k');
    cout << test[5];

    return 0;
}

the cout << test[5] returns k even though it not assigned to map before.How is it possible, did it do anything wrong , or is this the default behaviour of operand [] ? The above program creates keys with in intervals and assign the same value to them.For example test.assign(2,4, 'k') creates keys from to 2,3 and assign the value k to it.The idea is to prevent duplication of the values between keys generated by consecutive assign method.