diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index d6ac965..7153a56 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -314,7 +314,7 @@
 endfunc
 
 func CustomCompleteList(A, L, P)
-  return [ "Monday", "Tuesday", "Wednesday", {}]
+  return [ "Monday", "Tuesday", "Wednesday", {}, test_null_string()]
 endfunc
 
 func Test_CmdCompletion()