Punctuation: We promise [that] it's a big deal?
I'm editing a marketing campaign. Out loud, one might say, "We promise it's a big deal." I believe there's an unspoken "that" in there: "We promise [that] it's a big deal." Would you punctuate the sentence differently with the missing word? My first instinct was no punctuation: "We promise it's a big deal." But on subsequent passes, it started to look more and more incorrect. It's very possible that I've gotten way too close to the words and can't see the logical, simple answer in front of me! Thanks for your help.