diff --git a/pkgs/pict-pkgs/pict-doc/pict/scribblings/pict.scrbl b/pkgs/pict-pkgs/pict-doc/pict/scribblings/pict.scrbl
index 4ad936195c..e386f4b61e 100644
--- a/pkgs/pict-pkgs/pict-doc/pict/scribblings/pict.scrbl
+++ b/pkgs/pict-pkgs/pict-doc/pict/scribblings/pict.scrbl
@@ -288,7 +288,8 @@ override settings supplied by the context.
 @defproc*[([(ellipse [w real?] [h real?]) pict?]
            [(circle [diameter real?]) pict?]
            [(filled-ellipse [w real?] [h real?] [#:draw-border? draw-border? any/c #t]) pict?]
-           [(disk [diameter real?] [#:draw-border? draw-border? any/c #t]) pict?])]{
+           [(disk [diameter (and/c rational? (not/c negative?))]
+                  [#:draw-border? draw-border? any/c #t]) pict?])]{
 
 Unfilled and filled ellipses.
 
diff --git a/pkgs/pict-pkgs/pict-lib/pict/main.rkt b/pkgs/pict-pkgs/pict-lib/pict/main.rkt
index 03a3c09773..74487ab4af 100644
--- a/pkgs/pict-pkgs/pict-lib/pict/main.rkt
+++ b/pkgs/pict-pkgs/pict-lib/pict/main.rkt
@@ -9,7 +9,7 @@
              pict->argb-pixels
              argb-pixels->pict
              colorize
-             pin-under pin-over)
+             pin-under pin-over disk)
  (contract-out
   [colorize (-> pict? 
                 (or/c string? 
@@ -43,7 +43,8 @@
                    real?
                    (-> pict? pict? (values real? real?)))]
          [pict pict?])
-        [result pict?])]))
+        [result pict?])]
+  [disk (->* ((and/c rational? (not/c negative?))) (#:draw-border? any/c) pict?)]))
 
 (define (multiple-of-four-bytes? b)
   (zero? (modulo (bytes-length b) 4)))