diff --git a/src/vim9execute.c b/src/vim9execute.c
index 7485112..72cbb90 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1660,7 +1660,8 @@
     if (end_lnum > iptr->isn_lnum)
     {
 	ga_init2(&ga, sizeof(char_u *), 10);
-	for (lnum = iptr->isn_lnum; lnum < end_lnum; ++lnum)
+	for (lnum = iptr->isn_lnum; lnum < end_lnum
+				     && lnum <= ufunc->uf_lines.ga_len; ++lnum)
 	{
 	    char_u *p = ((char_u **)ufunc->uf_lines.ga_data)[lnum - 1];