OK, I'm confused again. The enclosed patch won't apply at all to 21.1. Kyle, is there a similar patch you want to apply for 21.1? I can't grok what's supposed to be going on from the limited context in the patch. The code you referenced does not appear in mapcar1 in 21.1. - vin Kyle> Jan Vroonhof writes: Kyle> Here's a patch that fixes this. Recommended for 21.1 and 21.2. Kyle> 1999-12-16 Kyle Jones <kyle_jones(a)wonderworks.com&gt; Kyle> * fns.c (mapcar1): when iterating over a list, check for Kyle> CONSP each iteration because bad user code might have Kyle> modified the list. Call Fcdr/Fcar if CONSP check fails Kyle> so that malformed lists cause an error to be signaled Kyle> instead of crashing. Kyle> --- src/fns.c 1999/10/24 03:48:39 1.30.2.23 Kyle> +++ src/fns.c 1999/12/17 04:13:14 Kyle> ＠＠ -3070,8 +3070,19 ＠＠ Kyle> { Kyle> for (i = 0; i < leni; i++) Kyle> { Kyle> - args[1] = XCAR (seq); Kyle> - seq = XCDR (seq); Kyle> + /* Use XCAR/XCDR to avoid function call overhead if we can. Kyle> + Otherwise call Fcar and Fcdr to get exceptions signaled Kyle> + if the Lisp code mangles the list. */ Kyle> + if (CONSP (seq)) Kyle> + { Kyle> + args[1] = XCAR (seq); Kyle> + seq = XCDR (seq); Kyle> + } Kyle> + else Kyle> + { Kyle> + args[1] = Fcar (seq); Kyle> + seq = Fcdr (seq); Kyle> + } Kyle> result = Ffuncall (2, args); Kyle> if (vals) vals[gcpro1.nvars++] = result; Kyle> }