diff --git a/util/protogen/main.go b/util/protogen/main.go
index 99f5b7b..fe89fbf 100644
--- a/util/protogen/main.go
+++ b/util/protogen/main.go
@@ -59,6 +59,7 @@ func emitMessage(g *protogen.GeneratedFile, msg *protogen.Message) {
 	g.P("//")
 	g.P("// Structures with the same field values have the same binary size.")
 	g.P("func (x *", msg.GoIdent.GoName, ") StableSize() (size int) {")
+	g.P("if x == nil { return 0 }")
 	if len(fs) != 0 {
 		for _, f := range fs {
 			if f.Desc.IsList() && marshalers[f.Desc.Kind()].RepeatedDouble {