slock segfaults

% gdb slock
GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
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Reading symbols from /usr/bin/slock...(no debugging symbols found)...done.
(gdb) run
Starting program: /usr/bin/slock
slock: cannot retrieve password entry (make sure to suid slock)
[Inferior 1 (process 22347) exited with code 01]
(gdb) bt full
No stack.
(gdb) set follow-fork-mode
Requires an argument. Valid arguments are child, parent.
(gdb) set follow-fork-mode 1
Undefined item: "1".
(gdb) set follow-fork-mode true
Undefined item: "true".
(gdb) set follow-fork-mode
child parent
(gdb) set follow-fork-mode child
(gdb) run
Starting program: /usr/bin/slock
slock: cannot retrieve password entry (make sure to suid slock)
[Inferior 1 (process 22503) exited with code 01]
(gdb) bt full
No stack.
(gdb) set follow-fork-mode parent
(gdb) run
Starting program: /usr/bin/slock
slock: cannot retrieve password entry (make sure to suid slock)
[Inferior 1 (process 22556) exited with code 01]
(gdb) bt full
No stack.
(gdb)

% ls -lha /usr/bin/slock
-rwsr-xr-x 1 root root 9.4K 2010-12-06 23:42 /usr/bin/slock

Same on Debian where it works