unset strangely rejects certain function names eg fu~

[root]

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-redhat-linux-gnu' 
-DCONF_VENDOR='redhat' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib  -D_GNU_SOURCE 
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -g -pipe -Wall 
-Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector 
--param=ssp-buffer-size=4 -m32 -march=i386 -mtune=generic 
-fasynchronous-unwind-tables
uname output: Linux trixbox1.local 2.6.18-8.1.14.el5 #1 SMP Thu Sep 27 18:58:54 
EDT 2007 i686 i686 i386 GNU/Linux
Machine Type: i686-redhat-linux-gnu

Bash Version: 3.1
Patch Level: 17
Release Status: release

Description:
        unset strangely rejects certain function names eg fu~

Repeat-By:
          $ function fun(){ echo running fun;}
          $ function fu~(){ echo running fu~;}
          $ fun
        running fun
          $ fu~
        running fu~
          $ unset fun
          $ unset fu~
        bash: unset: `fu~': not a valid identifier