diff --git a/run_electrum b/run_electrum
index 1f5ee2f03..bdfe723cb 100755
--- a/run_electrum
+++ b/run_electrum
@@ -352,6 +352,8 @@ if __name__ == '__main__':
         constants.set_regtest()
     elif config.get('simnet'):
         constants.set_simnet()
+    else:
+        raise Exception('lightning branch not available on mainnet')
 
     if cmdname == 'gui':
         configure_logging(config)