parameter expansion malinterpretation

[agathos]

Configuration Information [Automatically generated, do not change]:
Machine: i586
OS: linux-gnu
Compiler: i586-pc-linux-gnu-gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i586' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i586-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -O2 -march=k6-3 -pipe
uname output: Linux orome 2.6.19-gentoo-r5-FoBoS #1 Sun Feb 4 03:06:28 CET 2007 
i586 AMD-K6(tm) 3D+ Processor AuthenticAMD GNU/Linux
Machine Type: i586-pc-linux-gnu

Bash Version: 3.2
Patch Level: 9
Release Status: release

Description:
        When introducing negative values in a parameter or variable 
substitution dealing with substrings it does not interpret as a negative 
number, I think it interprets like a default value substitution

Repeat-By:
        agathos@orome ~ $ hello="hello"; echo ${hello:-1}
        hello
        agathos@orome ~ $ hello="hello"; echo ${hello:(-1)}
        o