[racket] confusing error message: what is "lang:read.1"?

Under certain circumstances, because of what I assume is a bug in my code, I get an error like this:

> define-values: assignment disallowed;
>  cannot re-define a constant
>   constant: lang:read.1
>   in module: "/Users/mb/git/racket/racket/collects/racket/main.rkt"
>   context...:
>    (submod /Users/mb/git/racket/racket/collects/racket/main.rkt reader): [running body]

I understand how the "assignment disallowed" error arises. What I don't understand is the meaning of "lang:read.1" in this context. I'm guessing it has something to do with the #lang line (?) but ... the rest is hazy. 

Clarification welcome.