English has largely lost the nominative/accusative distinction with the relative/interrogative pronouns "who" and "whom." The first person personal "I" and "me" are probably next. The nom./acc. distinction doesn't provide any useful information that isn't already conveyed by word order or prepositions. At this point, the distinction is a vestige of earlier forms of the language that were less analytic. And given that even decently educated native speakers fail to make this distinction, it's hard to make the case it's an actual error outside the register of formal writing.