Specify a range for a python command alias

Following the web page listed below I've created the following command alias in my .vimrc to conveniently prettify .json files:
command! Pj %!python -m json.tool

It works great but, ideally, I'd like to specify ranges. I tried the following:
command! -range=% Pj !python -m json.tool
But the command just hangs until such times as I Ctrl-C to abort.

I'm surprised by this since entering `3,4!python -m json.tool` at the command buffer works as expected.

What subtlety am I missing here?

Web page mentioned above:
https://pascalprecht.github.io/posts/pretty-print-json-in-vim

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