diff --git a/pkgs/gui-pkgs/gui-doc/scribblings/gui/miscwin-funcs.scrbl b/pkgs/gui-pkgs/gui-doc/scribblings/gui/miscwin-funcs.scrbl
index c4cad881d3..2da9618c39 100644
--- a/pkgs/gui-pkgs/gui-doc/scribblings/gui/miscwin-funcs.scrbl
+++ b/pkgs/gui-pkgs/gui-doc/scribblings/gui/miscwin-funcs.scrbl
@@ -321,10 +321,10 @@ Equivalent to @racket[(integer-in 1 1000000)].}
 
 
 @defproc[(register-collecting-blit [canvas (is-a?/c canvas%)]
-                                   [x real?]
-                                   [y real?]
-                                   [w (and/c real? (not/c negative?))]
-                                   [h (and/c real? (not/c negative?))]
+                                   [x position-integer?]
+                                   [y position-integer?]
+                                   [w dimension-integer?]
+                                   [h dimension-integer?]
                                    [on (is-a?/c bitmap%)]
                                    [off (is-a?/c bitmap%)]
                                    [on-x real? 0]
diff --git a/pkgs/gui-pkgs/gui-lib/mred/private/gdi.rkt b/pkgs/gui-pkgs/gui-lib/mred/private/gdi.rkt
index dd652a3d9f..bb068109fb 100644
--- a/pkgs/gui-pkgs/gui-lib/mred/private/gdi.rkt
+++ b/pkgs/gui-pkgs/gui-lib/mred/private/gdi.rkt
@@ -29,6 +29,8 @@
      [(canvas x y w h on off on-x on-y) (register-collecting-blit canvas x y w h on off on-x on-y 0 0)]
      [(canvas x y w h on off on-x on-y off-x) (register-collecting-blit canvas x y w h on off on-x on-y off-x 0)]
      [(canvas x y w h on off on-x on-y off-x off-y)
+      (define (check-real who v)
+        (unless (real? v) (raise-argument-error who "real?" v)))
       (check-instance 'register-collecting-blit canvas% 'canvas% #f canvas)
       (check-position 'register-collecting-blit x)
       (check-position 'register-collecting-blit y)
@@ -36,10 +38,10 @@
       (check-dimension 'register-collecting-blit h)
       (check-instance 'register-collecting-blit wx:bitmap% 'bitmap% #f on)
       (check-instance 'register-collecting-blit wx:bitmap% 'bitmap% #f off)
-      (check-position 'register-collecting-blit on-x)
-      (check-position 'register-collecting-blit on-y)
-      (check-position 'register-collecting-blit off-x)
-      (check-position 'register-collecting-blit off-y)
+      (check-real 'register-collecting-blit on-x)
+      (check-real 'register-collecting-blit on-y)
+      (check-real 'register-collecting-blit off-x)
+      (check-real 'register-collecting-blit off-y)
       (wx:register-collecting-blit (mred->wx canvas) x y w h on off on-x on-y off-x off-y)]))
 
   (define unregister-collecting-blit