diff --git a/collects/typed-scheme/private/free-variance.ss b/collects/typed-scheme/private/free-variance.ss
index 134c71400e..8ad421f757 100644
--- a/collects/typed-scheme/private/free-variance.ss
+++ b/collects/typed-scheme/private/free-variance.ss
@@ -59,7 +59,11 @@
    freess)
   ht)
 
-;; given a set of free variables, remove bound, add bound ...
+;; given a set of free variables, change bound to ...
+;; (if bound wasn't free, this will add it as Dotted
+;;  appropriately so that things that expect to see
+;;  it as "free" will -- fixes the case where the
+;;  dotted pre-type base doesn't use the bound).
 (define (fix-bound vs bound)
   (define vs* (hash-map* (lambda (k v) v) vs))
   (hash-set! vs* bound Dotted)