diff --git a/midend.c b/midend.c
index 03d6e98..5dd896f 100644
--- a/midend.c
+++ b/midend.c
@@ -63,8 +63,8 @@ struct midend {
     int nstates, statesize, statepos;
     struct midend_state_entry *states;
 
-    void *newgame_undo;
-    int newgame_undo_avail, newgame_undo_used;
+    void *newgame_undo_buf;
+    int newgame_undo_len, newgame_undo_size;
 
     game_params *params, *curparams;
     game_drawstate *drawstate;
@@ -158,8 +158,8 @@ midend *midend_new(frontend *fe, const game *ourgame,
     me->random = random_new(randseed, randseedsize);
     me->nstates = me->statesize = me->statepos = 0;
     me->states = NULL;
-    me->newgame_undo = 0;
-    me->newgame_undo_avail = me->newgame_undo_used = 0;
+    me->newgame_undo_buf = NULL;
+    me->newgame_undo_size = me->newgame_undo_len = 0;
     me->params = ourgame->default_params();
     me->game_id_change_notify_function = NULL;
     me->game_id_change_notify_ctx = NULL;
@@ -257,6 +257,7 @@ void midend_free(midend *me)
     if (me->drawing)
 	drawing_free(me->drawing);
     random_free(me->random);
+    sfree(me->newgame_undo_buf);
     sfree(me->states);
     sfree(me->desc);
     sfree(me->privdesc);
@@ -386,23 +387,22 @@ void midend_force_redraw(midend *me)
 static void newgame_serialise_write(void *ctx, void *buf, int len)
 {
     midend *const me = ctx;
-    int new_used;
+    int new_len;
 
-    assert(len < INT_MAX - me->newgame_undo_used);
-    new_used = me->newgame_undo_used + len;
-    if (new_used > me-> newgame_undo_avail) {
-	me->newgame_undo_avail = max(me->newgame_undo_avail, new_used);
-	me->newgame_undo_avail *= 2;
-	me->newgame_undo = sresize(me->newgame_undo,
-				   me->newgame_undo_avail, char);
+    assert(len < INT_MAX - me->newgame_undo_len);
+    new_len = me->newgame_undo_len + len;
+    if (new_len > me->newgame_undo_size) {
+	me->newgame_undo_size = new_len + new_len / 4 + 1024;
+	me->newgame_undo_buf = sresize(me->newgame_undo_buf,
+                                       me->newgame_undo_size, char);
     }
-    memcpy(me->newgame_undo + me->newgame_undo_used, buf, len);
-    me->newgame_undo_used = new_used;
+    memcpy(me->newgame_undo_buf + me->newgame_undo_len, buf, len);
+    me->newgame_undo_len = new_len;
 }
 
 void midend_new_game(midend *me)
 {
-    me->newgame_undo_used = 0;
+    me->newgame_undo_len = 0;
     midend_serialise(me, newgame_serialise_write, me);
 
     midend_stop_anim(me);
@@ -518,7 +518,7 @@ void midend_new_game(midend *me)
 
 int midend_can_undo(midend *me)
 {
-    return (me->statepos > 1 || me->newgame_undo_used);
+    return (me->statepos > 1 || me->newgame_undo_len);
 }
 
 int midend_can_redo(midend *me)
@@ -528,16 +528,16 @@ int midend_can_redo(midend *me)
 
 struct newgame_undo_deserialise_read_ctx {
     midend *me;
-    int size, pos;
+    int len, pos;
 };
 
-int newgame_undo_deserialise_read(void *ctx, void *buf, int len)
+static int newgame_undo_deserialise_read(void *ctx, void *buf, int len)
 {
     struct newgame_undo_deserialise_read_ctx *const rctx = ctx;
     midend *const me = rctx->me;
 
-    int use = min(len, rctx->size - rctx->pos);
-    memcpy(buf, me->newgame_undo + rctx->pos, use);
+    int use = min(len, rctx->len - rctx->pos);
+    memcpy(buf, me->newgame_undo_buf + rctx->pos, use);
     rctx->pos += use;
     return use;
 }
@@ -554,17 +554,15 @@ static int midend_undo(midend *me)
 	me->statepos--;
         me->dir = -1;
         return 1;
-    } else if (me->newgame_undo_used) {
+    } else if (me->newgame_undo_len) {
 	/* This undo cannot be undone with redo */
 	struct newgame_undo_deserialise_read_ctx rctx;
 	rctx.me = me;
-	rctx.size = me->newgame_undo_used; /* copy for reentrancy safety */
+	rctx.len = me->newgame_undo_len; /* copy for reentrancy safety */
 	rctx.pos = 0;
         deserialise_error =
 	    midend_deserialise(me, newgame_undo_deserialise_read, &rctx);
-	if (deserialise_error)
-	    /* umm, better to carry on than to crash ? */
-	    return 0;
+	assert(!deserialise_error);
 	return 1;
     } else
         return 0;
@@ -2130,7 +2128,7 @@ static char *midend_deserialise_internal(
      * more sophisticated way to decide when to discard the previous
      * game state.
      */
-    me->newgame_undo_used = 0;
+    me->newgame_undo_len = 0;
 
     {
         game_params *tmp;