diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index b3a8c49..68cbeb5 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -161,7 +161,8 @@
   END
   call v9.CheckLegacyAndVim9Success(lines)
 
-  call assert_fails('echo strcharpart("", 0, 0, {})', ['E728:', 'E728:'])
+  call assert_fails('call strcharpart("", 0, 0, {})', ['E728:', 'E728:'])
+  call assert_fails('call strcharpart("", 0, 0, -1)', ['E1023:', 'E1023:'])
 endfunc
 
 func Test_getreg_empty_list()