diff --git a/lib/paymentrequest.proto b/lib/paymentrequest.proto
index 6680810ea..32f9b56a1 100644
--- a/lib/paymentrequest.proto
+++ b/lib/paymentrequest.proto
@@ -6,6 +6,7 @@
 // https://github.com/bitcoin/bips/bip-0070/extensions.mediawiki
 //
 
+syntax = "proto2";
 package payments;
 option java_package = "org.bitcoin.protocols.payments";
 option java_outer_classname = "Protos";