"DNS did not resolve" error message on custom CNAME feature

When I try to enter the subdomain name pointing to bitbucket.org on the Custom domain page, I get the error message "DNS did not resolve." just below the CNAME box. This is different than the message I get when I enter an incorrect address: "The hostname "_________" does not resolve to bitbucket.org. Please check your DNS settings. Your hostname currently points to ___________."

The CNAME used to work, and I haven't touched the DNS records in months. A DNS propagation checker confirms that it has been set correctly on my end and propagated.

Either the error message is incorrect or it should be more specific as to what the problem is.