String Concatenation with $'\NNN' Error

[tony]

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: i686-pc-linux-gnu-gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -O2 -march=pentium3 -pipe
uname output: Linux steelbox 2.6.27.10 #11 Mon Mar 2 19:48:16 CST 2009 i686 AMD 
Athlon(tm) XP 3000+ AuthenticAMD GNU/Linux
Machine Type: i686-pc-linux-gnu

Bash Version: 3.2
Patch Level: 39
Release Status: release

Description:
        This command sequence concatenates a text string with a $'\NNN' string 
producing an error:
        shopt -s extglob; x="hello"$'\179'; echo "${x//+([^[:print:]])/?}"
        
        EXPECTED RESULT IS -> hello?

        HOWEVER, THE RESULT THAT IS PRODUCED IS -> hello?9

Repeat-By:
        Reproduce by running the above command sequence.