render file rename error

I am gettings this error after rendering my animation:

synfing(25640) [12:36:21 PM] error: synfig::save_canvas(): Unable to rename file to correct filename, errno=2

even though I have specified that my files should export as pngs, my files have a number filename.### instead of a file type extension.

You need to add .png to your render output filename - output.png will give you a sequence output.0001.png. Not adding .png gives errors.