diff --git a/src/undo.c b/src/undo.c
index b69f318..ba7c0b8 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -1787,7 +1787,7 @@
     linenr_T	line_lnum;
     colnr_T	line_colnr;
     linenr_T	line_count;
-    int		num_head = 0;
+    long	num_head = 0;
     long	old_header_seq, new_header_seq, cur_header_seq;
     long	seq_last, seq_cur;
     long	last_save_nr = 0;
@@ -1974,7 +1974,8 @@
      * When there are no headers uhp_table is NULL. */
     if (num_head > 0)
     {
-	uhp_table = (u_header_T **)U_ALLOC_LINE(
+	if (num_head < LONG_MAX / (long)sizeof(u_header_T *))
+	    uhp_table = (u_header_T **)U_ALLOC_LINE(
 					     num_head * sizeof(u_header_T *));
 	if (uhp_table == NULL)
 	    goto error;