I bought the album when it was new. It came with the lyrics in the jacket. It clearly reads Bruce don’t let me down.
A common mondegreen in the song is the perception that, following the title line, Jeff Lynne shouts "Bruce!" However, according to liner notes, he is actually saying a made-up word "Grroosss". This is similar to a German word for "greeting", Gruß, possibly referring to the Austrian and Bavarian greeting Grüß Gott that the group would have heard while recording the album in Munich. However, after the song's release, so many people had misinterpreted the word as "Bruce" that Jeff Lynne actually began to sing the word as "Bruce" for fun at live shows.