diff --git a/cue/build.go b/cue/build.go
index 3a8712b..5116825 100644
--- a/cue/build.go
+++ b/cue/build.go
@@ -30,6 +30,8 @@
 //
 // Any operation that involves two Values or Instances should originate from
 // the same Runtime.
+//
+// The zero value of a Runtime is ready to use.
 type Runtime struct {
 	ctx *build.Context // TODO: remove
 	idx *index