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Beugt man jemand oder niemand mit Endung?
The declension of indefinitve pronouns "jemand" or "niemand" seems not to follow strict rules. I can read all of the following:
Er hat mit jemandem gesprochen - Er hat mit jemand gesprochen
Sie hat niemanden gesehen - Sie hat niemand gesehen
Das ist niemandes Schuld - Das ist niemands Schuld
From this question I learned that not declining may be historically correct. Duden allows both variants not giving any help on what to prefer.
From a Google Ngram we see, that declined variants are increasingly used:
Answers here express that declension is felt to be appropriate in a written text. Are there still situations where using the non-declined forms are preferable?