diff --git a/lib/chibi/iset/constructors.scm b/lib/chibi/iset/constructors.scm
index 1f33222d..ec72f3e6 100644
--- a/lib/chibi/iset/constructors.scm
+++ b/lib/chibi/iset/constructors.scm
@@ -121,13 +121,13 @@
       (cond
        ;;         aaaa...
        ;; ...bbbb
-       ((< b-end a-start)
+       ((<= b-end a-start)
         (if (iset-should-merge-left? a b)
             (iset-merge-left! a b)
             (iset-adjoin-node-left! a b)))
        ;; ...aaaa
        ;;         bbbb...
-       ((> b-start a-end)
+       ((>= b-start a-end)
         (if (iset-should-merge-right? a b)
             (iset-merge-right! a b)
             (iset-adjoin-node-right! a b)))
diff --git a/tests/iset-tests.scm b/tests/iset-tests.scm
index 0bdb1b3b..e7b4b160 100644
--- a/tests/iset-tests.scm
+++ b/tests/iset-tests.scm
@@ -100,4 +100,7 @@
 (test-assert (iset<= (iset 97) (iset 97 117)))
 (test-assert (iset<= (iset 117) (iset 97 117)))
 
+(test-assert (iset= (iset-union (iset 1 3) (iset 3 4)) (iset 1 3 4)))
+(test-assert (iset= (iset-union (iset 3) (iset 1 3)) (iset 1 3)))
+
 (test-end)