diff --git a/collects/2htdp/private/img-err.ss b/collects/2htdp/private/img-err.ss
index 78ed2ca40e..f9f24ebc04 100644
--- a/collects/2htdp/private/img-err.ss
+++ b/collects/2htdp/private/img-err.ss
@@ -124,12 +124,19 @@
                 'non-negative-real-number
                 i arg)
      arg]
-    [(dx dy x1 y1 x2 y2 factor x-factor y-factor pull1 pull2)
+    [(dx dy x1 y1 x2 y2 pull1 pull2)
      (check-arg fn-name
                 (real? arg)
                 'real\ number
                 i arg)
      arg]
+    [(factor x-factor y-factor)
+     (check-arg fn-name
+                (and (real? arg)
+                     (positive? arg))
+                'positive\ real\ number
+                i arg)
+     arg]
     [(side-count)
      (check-arg fn-name
                 (side-count? arg)
diff --git a/collects/2htdp/tests/test-image.ss b/collects/2htdp/tests/test-image.ss
index 2372d1a0ec..604985cbbe 100644
--- a/collects/2htdp/tests/test-image.ss
+++ b/collects/2htdp/tests/test-image.ss
@@ -1512,10 +1512,11 @@
          (underlay image image)
          (underlay/xy image coord coord image)
          (crop coord coord size size image)
-         (scale/xy size size image)
-         (scale size image)
+         (scale/xy factor factor image)
+         (scale factor image)
          (rotate angle image))
-  
+
+  (factor (+ 1 big-nat) 1/2 1/3 1/4) ;; scaling factors
   (size big-nat)
   (mode 'outline 'solid "outline" "solid")
   (color "red" 'red "blue" "orange" "green" "black")
diff --git a/collects/teachpack/2htdp/scribblings/image.scrbl b/collects/teachpack/2htdp/scribblings/image.scrbl
index 0ef657b362..048a48f175 100644
--- a/collects/teachpack/2htdp/scribblings/image.scrbl
+++ b/collects/teachpack/2htdp/scribblings/image.scrbl
@@ -768,7 +768,7 @@ the parts that fit onto @scheme[scene].
           
 }
 
-@defproc[(scale [factor real?] [image image?]) image?]{
+@defproc[(scale [factor (and/c real? positive?)] [image image?]) image?]{
 
   Scales @scheme[image] by @scheme[factor]. 
   
@@ -788,7 +788,7 @@ the parts that fit onto @scheme[scene].
   
 }
 
-@defproc[(scale/xy [x-factor real?] [y-factor real?] [image image?]) image?]{
+@defproc[(scale/xy [x-factor (and/c real? positive?)] [y-factor (and/c real? positive?)] [image image?]) image?]{
   Scales @scheme[image] by @scheme[x-factor] horizontally and by 
   @scheme[y-factor] vertically.