diff --git a/lib/chibi/iset/constructors.scm b/lib/chibi/iset/constructors.scm
index bdbc4e3b..ba43c58c 100644
--- a/lib/chibi/iset/constructors.scm
+++ b/lib/chibi/iset/constructors.scm
@@ -81,7 +81,7 @@
           (if (let* ((left (iset-left iset))
                      (m-end (and left (iset-max-end left))))
                 (and m-end
-                     (or (< n m-end)
+                     (or (<= n m-end)
                          (< (- n m-end) s-diff))))
               (iset-adjoin1! (iset-left iset) n)
               (cond
@@ -98,7 +98,7 @@
           (if (let* ((right (iset-right iset))
                      (m-start (and right (iset-min-start right))))
                 (and m-start
-                     (or (> n m-start)
+                     (or (>= n m-start)
                          (> (- n m-start) e-diff))))
               (iset-adjoin1! (iset-right iset) n)
               (cond
diff --git a/tests/iset-tests.scm b/tests/iset-tests.scm
index 40fc076b..0c3c072c 100644
--- a/tests/iset-tests.scm
+++ b/tests/iset-tests.scm
@@ -1,6 +1,7 @@
 
 (cond-expand
- (modules (import (chibi iset) (chibi iset optimize) (srfi 1) (chibi test)))
+ (modules
+  (import (chibi) (chibi iset) (chibi iset optimize) (srfi 1) (chibi test)))
  (else #f))
 
 (test-begin "iset")
@@ -15,6 +16,7 @@
          ((0 1 2) (- 1) (- 2) (? 0))
          ((1 2 3 1000 2000) (u 1 4))
          ((1 2 3 1000 1005))
+         ((97308 97827 97845 97827))
          ((1 128 127))
          ((129 2 127))
          ((1 -128 -126))
@@ -25,10 +27,11 @@
   (for-each
    (lambda (tst)
      (let* ((ls (car tst))
-            (is (list->iset ls)))
+            (is (list->iset ls))
+            (ls2 (delete-duplicates ls =)))
        ;; initial creation and sanity checks
-       (test-assert (lset= equal? ls (iset->list is)))
-       (test (length ls) (iset-size is))
+       (test-assert (lset= equal? ls2 (iset->list is)))
+       (test (length ls2) (iset-size is))
        (for-each
         (lambda (x) (test-assert (iset-contains? is x)))
         ls)