I've seen this in a log message, resulting from using %s for printing a nil pointer of a type that implements SafeFormatter on a value receiver.

%!s(PANIC=SafeFormatter method: value method github.com/cockroachdb/cockroach/pkg/roachpb.Lease.SafeFormat called using nil *Lease pointer

This is different from what fmt.Printf does for the Stringer interface, where it prints <nil>. If possible, it'd certainly take the <nil>. I've found this thread that might describe how to handle this case in the logging lib: golang/go#20995