Why can't I use a literal Tab as the {commands} for :normal?

Question

I'd like to write the following code:

function Normal(command)
    execute "normal!" a:command
endfunction
call Normal("\<C-O>")
call Normal("\<C-I>")

The first call to Normal succeeds, but the second throws an error:

E471: Argument required

Indeed, you get the same error if you enter the command :norm! ^I with the keystrokes:

:norm!SpaceCtrl-VCtrl-IEnter

I have a vague idea that the problem might be something to do with the fact that a Tab character is whitespace, but I'd like to know more precisely what's going on.

N.B. I've already worked around the issue by adding a count "1\<C-I>", so this is purely out of curiosity.

Answer 1

If I guess then what's happening is, that the <c-i> gets expanded to a literal tab before inserting into the typeahead buffer. So Vim basically sees the command :norm! <tab>, which is not a valid command. I don't know a way around and even escaping did not work.

What works, is to use the feedkeys() command, e.g. :call feedkeys("\t", 'tn').