diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 327350b..6f3b81a 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -58,6 +58,9 @@
 
   call assert_equal(0, empty(function('Test_empty')))
   call assert_equal(0, empty(function('Test_empty', [0])))
+
+  call assert_fails("call empty(test_void())", 'E685:')
+  call assert_fails("call empty(test_unknown())", 'E685:')
 endfunc
 
 func Test_len()