diff --git a/collects/2htdp/image.rkt b/collects/2htdp/image.rkt
index 87b296e4ab..21593414c6 100644
--- a/collects/2htdp/image.rkt
+++ b/collects/2htdp/image.rkt
@@ -106,6 +106,7 @@ and they all have good sample contracts. (It is amazing what we can do with kids
          pen-style? 
          pen-cap?
          pen-join?
+         real-valued-posn?
          color-red color-blue color-green color? color
          pen-color pen-width pen-style pen-cap pen-join 
 
diff --git a/collects/2htdp/private/img-err.rkt b/collects/2htdp/private/img-err.rkt
index 7e42d26671..698bfcc213 100644
--- a/collects/2htdp/private/img-err.rkt
+++ b/collects/2htdp/private/img-err.rkt
@@ -10,6 +10,7 @@
          pen-style? 
          pen-cap?
          pen-join?
+         real-valued-posn?
          check-mode/color-combination)
 
 (require htdp/error
@@ -231,6 +232,10 @@
                      (andmap posn? arg))
                 'list-of-posns
                 i arg)
+     (check-arg fn-name
+                (andmap real-valued-posn? arg)
+                'list-of-posns-with-real-valued-x-and-y-coordinates
+                i arg)
      (check-arg fn-name
                 (>= (length arg) 3)
                 'list-of-at-least-three-posns
@@ -293,6 +298,10 @@
 (define (pen-join? arg)
   (member (if (string? arg) (string->symbol arg) arg)
           '(round bevel miter)))
+(define (real-valued-posn? arg)
+  (and (posn? arg)
+       (real? (posn-x arg))
+       (real? (posn-y arg))))
 
 
 ;; checks the dependent part of the 'color' specification
diff --git a/collects/2htdp/tests/test-image.rkt b/collects/2htdp/tests/test-image.rkt
index 89f844134f..644155295f 100644
--- a/collects/2htdp/tests/test-image.rkt
+++ b/collects/2htdp/tests/test-image.rkt
@@ -1914,6 +1914,11 @@
                    'solid (make-pen "black" 12 "solid" "round" "round"))
           =>
           #rx"^polygon: expected <image-color>")
+(test/exn (polygon (list (make-posn 0 0+1i) (make-posn 100 0) (make-posn 100 100))
+                   'solid (make-pen "black" 12 "solid" "round" "round"))
+          =>
+          #rx"^polygon: expected <list-of-posns-with-real-valued-x-and-y-coordinates>")
+
 
 (test/exn (save-image "tri.png" (triangle 50 "solid" "purple"))
           =>
diff --git a/collects/teachpack/2htdp/scribblings/image.scrbl b/collects/teachpack/2htdp/scribblings/image.scrbl
index 78ddc3eaca..1a7bb918fd 100644
--- a/collects/teachpack/2htdp/scribblings/image.scrbl
+++ b/collects/teachpack/2htdp/scribblings/image.scrbl
@@ -559,11 +559,11 @@ the @scheme[point-count] argument determines how many points the star has.
 }
 
                      
-@defproc*[([(polygon [vertices (listof posn?)] 
+@defproc*[([(polygon [vertices (listof real-valued-posn?)] 
                      [mode mode?]
                      [color image-color?])
             image?]
-           [(polygon [vertices (listof posn?)] 
+           [(polygon [vertices (listof real-valued-posn?)] 
                      [outline-mode (or/c 'outline "outline")]
                      [pen-or-color (or/c pen? image-color?)])
             image?])]{
@@ -1266,6 +1266,11 @@ Using @racket["pinhole"] or @racket['pinhole] is only allowed when all of the im
   Determines if @racket[x] is an integer greater than or equal to @racket[1].                                           
 }
 
+@defproc[(real-valued-posn? [x any/c]) boolean?]{
+  Determines if @racket[x] is a @racket[posn] whose @racket[_x] and @racket[_y]
+  fields are both @racket[real?] numbers.
+}
+
 @defstruct[pen ([color image-color?]
                 [width (and/c real? (<=/c 0 255))]
                 [style pen-style?]