diff --git a/revid/revid.go b/revid/revid.go
index bec75d7f..92102143 100644
--- a/revid/revid.go
+++ b/revid/revid.go
@@ -392,6 +392,7 @@ func (r *Revid) Update(vars map[string]string) error {
 	for key, value := range vars {
 		switch key {
 		case "Output":
+			r.config.Outputs = make([]uint8, 1)
 			// FIXME(kortschak): There can be only one!
 			// How do we specify outputs after the first?
 			//