diff --git a/src/testdir/test_vartabs.vim b/src/testdir/test_vartabs.vim
index 3596722..95fffe0 100644
--- a/src/testdir/test_vartabs.vim
+++ b/src/testdir/test_vartabs.vim
@@ -369,6 +369,8 @@
   let lines = ScreenLines([1, 3], winwidth(0))
   call s:compare_lines(expect4, lines)
 
+  call assert_fails('call shiftwidth([])', 'E745:')
+
   " cleanup
   bw!
   bw!