0.0% Engine Efficiency? (Potential bug report)

While working on a Kei engine, I ran into a display of 0% engine efficiency; on an engine and car that are throwing no errors and seem to be otherwise working fine.

Could this be a bug, or is there something I’m misunderstanding?

Given that there are no other abnormal readouts, I suspect that this is definitely a bug, and not something to be expected during the engine design phase - it could be the result of a calculation error when determining the efficiency rating.