If you do not have the following menu item: "Keychain Access->Certificate Assistant->Request a Certificate from a Certificate Authority", then you need a new WWDR Intermediate Certificate. My menu item was originally similar, but had some extra text in the title. Downloading and installing a new certificate changed the menu item and allowed generation of a valid CSR. This was odd because my previous WWDR Intermediate Certificate appeared in my keychain as an unexpired certificate.
I logged a bug at bugreport.apple.com asking for a clearer error message and pointing back to this SO discussion. Edit: fyi.. apparently developer portal bugs should be logged by sending email to devprograms@apple.com instead of the "product" targeted bugreport website. I encourage folks that have wasted time on this issue to provide helpful feedback regarding the issue via email.