Make synstack() work on the character just after the end of the line.
diff --git a/src/eval.c b/src/eval.c
index a415849..2944758 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -17064,7 +17064,7 @@
col = get_tv_number(&argvars[1]) - 1; /* -1 on type error */
if (lnum >= 1 && lnum <= curbuf->b_ml.ml_line_count
- && col >= 0 && (col == 0 || col < (long)STRLEN(ml_get(lnum)))
+ && col >= 0 && col <= (long)STRLEN(ml_get(lnum))
&& rettv_list_alloc(rettv) != FAIL)
{
(void)syn_get_id(curwin, lnum, (colnr_T)col, FALSE, NULL, TRUE);