diff --git a/cube/Makefile b/cube/Makefile
index 7b2041f..9702f96 100644
--- a/cube/Makefile
+++ b/cube/Makefile
@@ -22,6 +22,9 @@
 
 # NETWORK = LAN
 
+ifndef NETWORK
+$(error NETWORK is not set)
+endif
 
 ######################################
 # target