I don't understand the reason why you need accusative instead of nominative after the impersonal construct es gibt (or gibt es in the questions).
According to the analysis of the parts of speech, what comes after it should be the subject of the sentence, not the object, like for instance in:
Es gibt ein(en) Fehler auf dem Tisch.
Can you clarify the use of accusative here? I can probably deduce that the subject is "es" so that Fehler would become an object, but I'd like to have the complete grammatical analysis.