tab doesn't work within incremental-complete-word

[Han Pingtian <hanpt@xxxxxxxxxxxxxxxxxx>]

Hi,

Looks like tab (complete-word) doesn't work within
incremental-complete-word on my system. 

I have initialised incremental-complete-word by putting those in my .zshrc:

zstyle :incremental list yes
autoload -U incremental-complete-word
zle -N incremental-complete-word
bindkey '^xi' incremental-complete-word

But after activiated by '^xi' after inputting 'ls', I cannot use tab to
complete files in current directory:

% ls
aa  bb  cc
% ls	# ctrl-x i works, but tab doesn't
incremental (complete): -no prefix-
files
aa  bb  cc