I don’t mean to sound insulting, I’ve done this myself quite a few times.
Double- and triple-check that you’re not hitting the wrong key in the process? Heaven knows how many times I’ve hit the comma instead of the period – and still can’t see it for looking after I get an “invalid format” rejection. :o Eventually (hopefully) the light DOES finally dawn so I can quit blushing and cussing and do it right. 
And who knows, maybe someday the Internet will fall into line with what’s left of my brain and we WILL have comma-com and comma-net addresses accepted as valid. 