Using HLOOKUP to encode a message returns an error message for punctuation marks

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I have a few sentences that include punctuation. I'm trying to code a message that my students will decode using matrices.

The HLOOKUP function is fine with letters. I've dropped in periods ( . ) for all spaces between words. There are also a few ? and – and ' and , marks. HOOKUP returns errors for all of them.

I've tried formatting the cells as General and Text and the results are the same.