def _check_required_fields(self):
        """
        Make sure, that internal dictionary contains all fields, which are
        required by the webform.
        """
        assert self._POST["P0501010__a"] != "", "ISBN is required!"

        # export script accepts only czech ISBNs
        for isbn_field_name in ("P0501010__a", "P1601ISB__a"):
            check = PostData._czech_isbn_check(self._POST[isbn_field_name])
            assert check, "Only czech ISBN is accepted!"

        assert self._POST["P1601ISB__a"] != "", "Hidden ISBN field is required!"

        assert self._POST["P07012001_a"] != "", "Nazev is required!"
        assert self._POST["P0901210__a"] != "", "Místo vydání is required!"

        assert self._POST["P0903210__d"] != "", "Datum vydání is required!"
        assert self._POST["P0801205__a"] != "", "Pořadí vydání is required!"

        # Zpracovatel záznamu
        assert self._POST["P1501IST1_a"] != "", "Zpracovatel is required! (H)"
        assert self._POST["P1502IST1_b"] != "", "Zpracovatel is required! (V)"

        # vazba/forma
        assert self._POST["P0502010__b"] != "", "Vazba/forma is required!"

        # assert self._POST["P110185640u"] != "", "URL is required!"

        # Formát (pouze pro epublikace)
        if self._POST["P0502010__b"] == FormatEnum.ONLINE:
            assert self._POST["P0503010__x"] != "", "Format is required!"

        assert self._POST["P0902210__c"] != "", "Nakladatel is required!"

        def to_unicode(inp):
            try:
                return unicode(inp)
            except UnicodeDecodeError:
                return unicode(inp, "utf-8")

        # check lenght of annotation field - try to convert string to unicode,
        # to count characters, not combination bytes
        annotation_length = len(to_unicode(self._POST["P1001330__a"]))
        annotation_length -= len(to_unicode(ANNOTATION_PREFIX))
        assert annotation_length <= 500, "Annotation is too long (> 500)."
Make sure, that internal dictionary contains all fields, which are
        required by the webform.