Don't feel bad because even those of us who've been doing this for ages will still occasionally sit there and blank out despite the error message explicitly telling what's wrong. It's only obvious after you've bothered to show it to someone else. And you read it again. And then sit down and actually read it. And then there's the "Derp."

If you ever have a debugging problem like this where you're not seeing the problem, go walk off for 5-10 minutes and do something unrelated. When you come back and look at it fresh it can help.