In a fresh org file I try this:

#+begin_src scheme :session mainsession

(define (mysquare x)

(* x x))

#+end_src

and get this:

executing Scheme code block...

=> #<void>

: ‘org-babel-script-escape’ expects a string

I'm using latest elpa everything (orgmode, geiser) with Racket v6.10.1 as of this morning. Just started happening. . . .