Right you are, of course. Zep was notorious on their early albums for making uncredited use of classic blues lyrics and riffs. I referenced them rather than Johnson figuring that more people would have heard their cover than the original.
Wikipedia mentions that the line may not actually be original with Johnson:
...parts of Johnson's "Travelling Riverside Blues" are used as lyrics in Led Zeppelin's "The Lemon Song", namely the "squeeze my lemon" sequence. It is likely that Johnson borrowed this himself, from a song recorded in the same year (1937) called "She Squeezed My Lemon", by Roosevelt Sykes.http://en.wikipedia.org/wiki/Travelling_Riverside_Blues
Huh! Thanks - I didn’t know that!
Now I’m going to see if I can find a mp3 of the Sykes song.