It looks like a bug, that with action in templates produce error with missingkey=error.
Due the documentation with action is made for checking if pipeline is empty, and perfectly fits in logic, for example, if foo has key "bar", then print something. But with missingkey=error option it fails all template generation with error. (http://play.golang.org/p/kmmkkrHZYI)
I found a workaround for this case, using index function. It works as expected, and does not produce errors on not existent key. But it makes templates less readable, and, I think, simple addressing should have same behaviour.