diff --git a/xwords4/common/device.c b/xwords4/common/device.c
index 96bd28319..3ae7f78a2 100644
--- a/xwords4/common/device.c
+++ b/xwords4/common/device.c
@@ -382,6 +382,7 @@ dvc_makeMQTTMessages( XW_DUtilCtxt* dutil, XWEnv xwe,
         for ( SendMsgsPacket* packet = (SendMsgsPacket*)msgs;
               !!packet; packet = (SendMsgsPacket* const)packet->next ) {
             XP_U32 len = packet->len;
+            XP_ASSERT( 0 < len ); /* will confuse server */
             stream_putU32VL( stream, len );
             stream_putBytes( stream, packet->buf, len );
             nSent1 += len;