diff --git a/collects/scheme/private/string.ss b/collects/scheme/private/string.ss
index 731833332b..0d04dcaeab 100644
--- a/collects/scheme/private/string.ss
+++ b/collects/scheme/private/string.ss
@@ -12,18 +12,22 @@
   (require (for-syntax "stxcase-scheme.ss"))
 
   ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
   (define (real->decimal-string n [digits 2])
-    (let* ([e (expt 10 digits)]
-           [num (round (abs (* e (inexact->exact n))))])
-      (format "~a~a.~a"
-              (if (negative? n) "-" "")
-              (quotient num e)
-              (let ([s (number->string (remainder num e))])
-                (if (= (string-length s) digits)
-                  s
-                  (string-append (make-string (- digits (string-length s)) #\0)
-                                 s))))))
+    (unless (exact-nonnegative-integer? digits)
+      (raise-type-error 'real->decimal-string "exact-nonnegative-integer" n))
+    (if (zero? digits)
+      (number->string (inexact->exact (round n)))
+      (let* ([e (expt 10 digits)]
+             [num (round (abs (* e (inexact->exact n))))])
+        (format "~a~a.~a"
+                (if (negative? n) "-" "")
+                (quotient num e)
+                (let ([s (number->string (remainder num e))])
+                  (if (= (string-length s) digits)
+                    s
+                    (string-append (make-string (- digits (string-length s)) #\0)
+                                   s)))))))
+
   ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;; Regexp helpers
 
diff --git a/collects/tests/mzscheme/string.ss b/collects/tests/mzscheme/string.ss
index 8ace942d04..0948978835 100644
--- a/collects/tests/mzscheme/string.ss
+++ b/collects/tests/mzscheme/string.ss
@@ -5,6 +5,20 @@
 
 ;; to add when this library is there: (require scheme/string)
 
+(test "0" real->decimal-string 0 0)
+(test "0" real->decimal-string 0.0 0)
+(test "1" real->decimal-string 0.6 0)
+(test "1" real->decimal-string 3/4 0)
+(test "1" real->decimal-string 1.2 0)
+(test "0" real->decimal-string -0.0 0) ; !
+(test "-1" real->decimal-string -0.6 0)
+(test "-1" real->decimal-string -3/4 0)
+(test "-1" real->decimal-string -1.2 0)
+(test "1.20" real->decimal-string 1.2)
+(test "-1.20" real->decimal-string -1.2)
+(test "1.00" real->decimal-string 0.99999999999)
+(test "-1.00" real->decimal-string -0.99999999999)
+
 (let ([s (list->string
 	  (let loop ([i 0])
 	    (if (= i 256)