I am receiving this assertion failed error when trying to insert an element in a stxxl map.

The entire assertion error is the following:

resCache: /usr/include/stxxl/bits/containers/btree/btree.h:470: std::pair >, bool> stxxl::btree::btree::insert(const value_type&) [with KeyType = e_my_key, DataType = unsigned int, CompareType = comp_type, unsigned int RawNodeSize = 16384u, unsigned int RawLeafSize = 131072u, PDAllocStrategy = stxxl::SR, stxxl::btree::btree::value_type = std::pair]: Assertion `it != root_node_.end()' failed. Aborted

Any idea?

Edit: Here's the code fragment

void request_handler::handle_request(my_key& query, reply& rep)
{
    c_++;

    strip(query.content);
    std::cout << "Received query " << query.content << " by thread " << boost::this_thread::get_id() << ". It is number " << c_ << "\n";
    strcpy(element.first.content, query.content);
    element.second = c_;
    testcache_.insert(element);

    STXXL_MSG("Records in map: " << testcache_.size());

}

Edit2 here's more details (I omit constants, e.g. MAX_QUERY_LEN)

struct comp_type : std::binary_function<my_key, my_key, bool>
{
    bool operator () (const my_key & a, const my_key & b) const
    {
            return strncmp(a.content, b.content, MAX_QUERY_LEN) < 0;
    }
    static my_key max_value()
    {
            return max_key;
    }
    static my_key min_value()
    {
            return min_key;
    }
};

typedef stxxl::map<my_key, my_data, comp_type> cacheType;


cacheType testcache_;

request_handler::request_handler()
:testcache_(NODE_CACHE_SIZE, LEAF_CACHE_SIZE)
{
    c_ = 0;
    memset(max_key.content, (std::numeric_limits<unsigned char>::max)(), MAX_QUERY_LEN);
    memset(min_key.content, (std::numeric_limits<unsigned char>::min)(), MAX_QUERY_LEN);

    testcache_.enable_prefetching();

    STXXL_MSG("Records in map: " << testcache_.size());
}