diff --git a/keystore/keystore_client.proto b/keystore/keystore_client.proto
index cd520dc..cbafd54 100644
--- a/keystore/keystore_client.proto
+++ b/keystore/keystore_client.proto
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+syntax = "proto2";
+
 package keystore;
 
 option optimize_for = LITE_RUNTIME;