diff --git a/groupcachepb/groupcache.proto b/groupcachepb/groupcache.proto
index 153e851..b5bdff9 100644
--- a/groupcachepb/groupcache.proto
+++ b/groupcachepb/groupcache.proto
@@ -14,6 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+syntax = "proto2";
+
 package groupcachepb;
 
 message GetRequest {