[racket] define-values: cannot re-define a constant: string-trim

This error:
define-values: cannot re-define a constant: string-trim
has just started to appear whenever I try to modify in the lower pane a 
function that I've defined in the upper pane.  I can work around it by 
using a different name, but it makes it quite difficult to do 
incremental development.  Also, it seems to be a new error.  (I.e., one 
that wasn't there earlier today.)

Anybody have any idea what's going on?