trace failure on x86-64

First of all let me say how great it is at last to have working ports of GNU 
Prolog to MacOS/x86 and Linux/x86-64!

However I am having a few problems with the x86-64 port. I was getting some 
strange behaviour, and tried to pin it down with the 'trace' facility. But 
this caused fresh failures, for example as follows:

$ gprolog
GNU Prolog 1.3.0
By Daniel Diaz
Copyright (C) 1999-2007 Daniel Diaz
| ?- length([a,s,d],X).

X = 3

yes
| ?- trace.
The debugger will first creep -- showing everything (trace)

yes
{trace}
| ?- length([a,s,d],X).

no
{trace}
| ?-

This is under Fedora Core 6 for x86-64, compiled with gcc 4.1.1 and running on 
a MacBook Mk2 (Intel core 2 duo). 

Thanks for any help!
	--Jasper