"nmap" issue

Hi,
I'm trying to create a mapping to maximize current window. But somehow
it only executed portion of my mapping.
My mapping is:
nmap <Esc>m <C-W>_<C-W>|
Looks like after I press <Esc>m , it only executes <C-W>_ .
And if we change it to:
nmap <Esc>m <C-W>|<C-W>
It give me the following error when I source .vimrc :
Error detected while processing /usr2/jianzhen/.vimrc:
line 500:
E488: Trailing characters: <C-W>_

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Best Regards
Jerry Dai

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