diff --git a/luprex/cpp/drv/driver-common.cpp b/luprex/cpp/drv/driver-common.cpp
index 2affc465..d005e606 100644
--- a/luprex/cpp/drv/driver-common.cpp
+++ b/luprex/cpp/drv/driver-common.cpp
@@ -270,6 +270,7 @@ class Driver {
         char *data;
         int ndata = BIO_get_mem_data(chan.send_bio, &data);
         if (ndata > DRV_SHORTSTRING_SIZE) ndata = DRV_SHORTSTRING_SIZE;
+        assert(ndata > 0);
         std::string err;
         int nwrote = socket_send(chan.socket, data, ndata, err);
         // std::cerr << "chan " << chan.chid << " send " << nwrote << " err=" << err << std::endl;