Nil is not an error, much less a serious one.

cardsim/errors.go

@@ -55,6 +55,9 @@ func Warningf(f string, args ...any) *Warning {
 // IsSeriousError returns whether e is a non-warning error. If e is nil, this
 // returns false.
 func IsSeriousError(e error) bool {
+	if e == nil {
+		return false
+	}
 	return !errors.Is(e, AnyWarning)
 }