Segmentation fault on append to Glib::ustring

[Spazzatura.Live <kharhonte hotmail com>]

Probably this is more a libcurl question than a gtkmm one, but i saw an 
example in which a std::string works so:

When i let curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, 
writefunction), on my ustring:

    int writefunction(char *data, size_t size, size_t nmemb, 
Glib::ustring *buffer)
    {
        if(buffer!=NULL)
        {
            buffer->append(data, size*nmemb);
            return size*nmemb;
        }
        return 0;
    }

it gives always a segmentation fault.

Attached the code.

Attachment: code.zip
Description: Zip archive