diff --git a/testpb/test.proto b/testpb/test.proto
index 7292dce..b9dc6c9 100644
--- a/testpb/test.proto
+++ b/testpb/test.proto
@@ -14,6 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+syntax = "proto2";
+
 package testpb;
 
 message TestMessage {