patch 8.2.3916: no error for passing an invalid line number to append() Commit: https://github.com/vim/vim/commit/8dac2acd6a79d571ff5409d9c90b4c9e73237eb4 Author: Bram Moolenaar <Bram@vim.org> Date: Mon Dec 27 20:57:06 2021 +0000 patch 8.2.3916: no error for passing an invalid line number to append() Problem: No error for passing an invalid line number to append(). Solution: In Vim9 script check for a non-negative number. (closes https://github.com/vim/vim/issues/9417)

--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -151,6 +151,8 @@ set_buffer_lines(
     if (buf == NULL || (!is_curbuf && buf->b_ml.ml_mfp == NULL) || lnum < 1)
     {
 	rettv->vval.v_number = 1;	// FAIL
+	if (in_vim9script() && lnum < 1)
+	    semsg(_(e_invalid_line_number_nr), lnum_arg);
 	return;
     }