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I've built a bird for a game I'm working on, and when I tried to export the beak mesh as a .fbx file it gave me this error. 
I've applied all the modifiers to the mesh already.
Here's the file https://drive.google.com/file/d/1Ah650tjjbcC3zb4Rw8fqIuto7U9OsTLZ/view?usp=sharing
I've solved the problem, you have to smart uv project the mesh before exporting.
