@Tim Edwards i have filed this ngspice bug: https://sourceforge.net/p/ngspice/bugs/596/ Have you ever seen this odd behaviour? it happens to me from time to time, and can't solve this.

I haven't run across this one before (also, I saw Holger's reply, so I assume that's a valid workaround?).

Yes, the remzerovec removes vectors with no values (such as identically 0 vectors, i guess) and the error is gone.