diff --git a/rtu/rtu.lua b/rtu/rtu.lua
index 49984aa..f86d00a 100644
--- a/rtu/rtu.lua
+++ b/rtu/rtu.lua
@@ -66,6 +66,7 @@ function rtu.validate_config(cfg)
     cfv.assert_min(cfg.ConnTimeout, 2)
     cfv.assert_type_bool(cfg.WirelessModem)
     cfv.assert((cfg.WiredModem == false) or (type(cfg.WiredModem) == "string"))
+    cfv.assert(cfg.WirelessModem or (type(cfg.WiredModem) == "string"))
     cfv.assert_type_bool(cfg.PreferWireless)
     cfv.assert_type_num(cfg.TrustedRange)
     cfv.assert_min(cfg.TrustedRange, 0)