Problems with GNAT

[Tim <blandtg@dcs.gla.ac.uk>]

I have been writing bits of Ada code on a UNIX system. 
I have developed a package of utilities for sound generation. 

Upon trying to run a driver program (packtest.adb) I had written to 
exercise this package of facilities I kept getting a constraint error. 

In attempting to locate this bug I used gdb. 
Which eventually came up with:-
(gdb) step
__gnat_raise_constraint_error () at a-raise.c:154
a-raise.c:154: No such file or directory.
Current language:  auto; currently c
(gdb) where
#0  __gnat_raise_constraint_error () at a-raise.c:154
#1  0x13bd8 in packtest () at packtest.adb:64
#2  0x236c in main (argc=1, argv=0xeffff97c) at b_packtest.c:35

Does anybody know what is going on here? 
I had gdb running fine with Ada as the language and 
then it hits this error and goes phut. 
Any suggestions?
Thanks in anticipation
Tim