yesod add-handler results in error

I followed the 6 steps outlined here http://ift.tt/RwLk8n (except the first step, b/c I already had stack), and everything works. However, when I attempt to run stack exec -- yesod add-handler, I get the following output:

Name of route (without trailing R): A
Enter route pattern (ex: /entry/#EntryId): /a
Enter space-separated list of methods (ex: GET POST): GET
yesod: Application.hs: openFile: does not exist (No such file or directory)

I also tried it with the yesod-postgres stack template, and got the same result. Any ideas?