mrdoob/three.js — pull request #5389 by dmtaub (2014-09-30 14:31:50+00:00)
Title: adding check for uv[index] undefined in geometry merge. buffer geometry ...

When using fromBufferGeometry, converting leaves objects liked [undefined, undefined, undefined] so this is a stopgap to allow merge until that is fixed.
There is also sometimes a problem with normals, that I haven't found an appropriate fix for. In another branch, I was fixing it by simply creating an empty attribute array for both UVs and Normals.