It's a performance error; the predicate should be nominative.
English pronoun cases don't divide up the same way Esperanto cases do (e.g., prepositions take the object case), but note that many English speakers say, "It is me" rather than "It is I". (I don't know Emile's first language.)
Also, leaving off the accusative ending is such a pitfall for most beginners at Esperanto that people sometimes overcorrect anything matching the pattern "nominative verb nominative" to "nominative verb accusative".
Edit: Corrected "pronouns take" to "prepositions take".
English pronoun cases don't divide up the same way Esperanto cases do (e.g., prepositions take the object case), but note that many English speakers say, "It is me" rather than "It is I". (I don't know Emile's first language.)
Right. I forget the technical terms used in the case of English (they aren't usually called nominative vs. accusative anymore), but the default case is 'me': 'I' is the special case, used only in subject position.
(The default case in English is actually descended from the Old English dative, not the accusative, ...
I just read this in the Wikipedia article on the Mattaponi River and it really tickled me. If only all language were so rational!