"I thought the RSVP questions were just like, 'Are you coming? Yes or no?' I didn't realise that it's like, 'What size bed do you want?'" he said.
After receiving a number of strange RSVPs back, Johnson said he realised their mistake.
"The dress code is set to 'something sexy', so that's also happening," laughed Byrne.
The couple is set to get married later this year, after postponing their wedding for 12 months amid the coronavirus pandemic.
Johnson proposed in April 2019, and the pair has since welcomed two daughters: Marlie-Mae, in June 2019, and Lola, in February this year.
"We had originally thought that we would have our wedding this year, but juggling a baby and full-time work meant that we never got around to locking in a date and venue," Byrne told TVWeek last April.
"Luckily our lack of plans and disorganisation meant that we didn't have to cancel anything. We are now looking to 2021."
In September, Johnson told The Wedding Digest podcast that he and Byrne originally planned to tie the knot in 2020.
"I'm going to say it's down to the fact that I'm very unorganised that we didn't, but it kind of worked out in our favour though, because if we had planned to get married this year, then those plans would have been absolutely obliterated because of covid," he explained.
The proud dad added that juggling work with parenting meant that the wedding was "pushed to the bottom of the to-do list" but said their eldest daughter will play a role in the nuptials.
"I think she'll be old enough to be able to take on the responsibility of being flower girl. I'm going to be a blubbering mess. I get a little bit emotional."
