diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 4b6774f..3c15101 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -142,6 +142,7 @@
   let b2 = b1[:]
   call assert_true(b1 == b2)
   call assert_false(b1 is b2)
+  call assert_true(b1 isnot b2)
 
   call assert_fails('let x = b1 > b2')
   call assert_fails('let x = b1 < b2')