Guest, MCP would seem to be right on this account.

In Deena Rosenberg's book, "Fascinating Rhythm: the Collaboration of George and Ira Gershwin", the author lists the lyricist for "Summertime" as DuBose Heyward. Some of the lyrics for "Porgy and Bess" were written/edited by Ira Gershwin, but all of the lyrics for that particular song were by Heyward. The reason for incorrect lyric attributions in such a case is probably that, when originally published, the names of all the lyricists for the show appeared on the music, not just the lyricist for that song.

My versions, in both Ms. Rosenberg's book and Sheet Music Magazine, agree with MCP's version of the lyric. (Although Sheet Music perpetuates the incorrect attribution)

This may be my first intelligent contribution to this website!