diff --git a/collects/mzlib/foreign.ss b/collects/mzlib/foreign.ss
index c034b4b748..0d320edcec 100644
--- a/collects/mzlib/foreign.ss
+++ b/collects/mzlib/foreign.ss
@@ -1013,13 +1013,13 @@
   (list->cvector args type))
 
 (define* (cvector-ref v i)
-  (if (and (integer? i) (<= 0 i (sub1 (cvector-length v))))
+  (if (and (exact-nonnegative-integer? i) (< i (cvector-length v)))
     (ptr-ref (cvector-ptr v) (cvector-type v) i)
     (error 'cvector-ref "bad index ~e for cvector bounds of 0..~e"
            i (sub1 (cvector-length v)))))
 
 (define* (cvector-set! v i x)
-  (if (and (integer? i) (<= 0 i (sub1 (cvector-length v))))
+  (if (and (exact-nonnegative-integer? i) (< i (cvector-length v)))
     (ptr-set! (cvector-ptr v) (cvector-type v) i x)
     (error 'cvector-ref "bad index ~e for cvector bounds of 0..~e"
            i (sub1 (cvector-length v)))))
@@ -1076,14 +1076,14 @@
                (list->TAG vals))
              (define* (TAG-ref v i)
                (if (TAG? v)
-                   (if (and (integer? i) (< -1 i (TAG-length v)))
+                   (if (and (exact-nonnegative-integer? i) (< i (TAG-length v)))
                        (ptr-ref (TAG-ptr v) type i)
                        (error 'TAG-ref "bad index ~e for ~a bounds of 0..~e"
                               i 'TAG (sub1 (TAG-length v))))
                    (raise-type-error 'TAG-ref TAGname v)))
              (define* (TAG-set! v i x)
                (if (TAG? v)
-                   (if (and (integer? i) (< -1 i (TAG-length v)))
+                   (if (and (exact-nonnegative-integer? i) (< i (TAG-length v)))
                        (ptr-set! (TAG-ptr v) type i x)
                        (error 'TAG-set! "bad index ~e for ~a bounds of 0..~e"
                               i 'TAG (sub1 (TAG-length v))))