You should use d3.geo.bounds for this, as you need bounding box in spherical coordinates. (Using d3.geo.path with the identity function as a projection doesnâ€™t work because d3.geo.path can only compute a bounding box in Cartesian coordinates.) See Jason Daviesâ€™ Geographic Bounding Boxes page for details....

(This is an interesting question) I can't quite decipher what's at play here, but FWIW, this is the implementation of the Lagrange invert() method. Evidently, there are cases when it returns null, presumably whenever x or y are outside some extents (x,y are pre-transformed here). So I guess you're supposed...