diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index e2c332a..a29e2e6 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -291,6 +291,7 @@
   call assert_equal(3, index(0z11110111, 0x11, -2))
   call assert_equal(0, index(0z11110111, 0x11, -10))
   call assert_fails("echo index(0z11110111, 0x11, [])", 'E745:')
+  call assert_equal(-1, index(test_null_blob(), 1))
 
   call assert_fails('call index("asdf", 0)', 'E897:')
 endfunc