Is it by design, that d3.geoIdentity() doesn't implement projection.center?

Using Mike's example code for projection transition at
https://stackoverflow.com/questions/17045826/how-to-do-smooth-transition-for-map-reprojection-in-d3-js
I ran into problems transitioning from an identity projection.
Was easily fixed by disabling all references to projection.center
but wanted to understand what I am missing here.

Thanks!