diff --git a/bacnet-stack/datalink.c b/bacnet-stack/datalink.c
index 77df8ba1..b62d661a 100644
--- a/bacnet-stack/datalink.c
+++ b/bacnet-stack/datalink.c
@@ -39,6 +39,12 @@
 // However, it could handle multiple data links with the
 // addition of passing a network number or datalink number
 // as part of the calls.
+#if defined(BACDL_ARCNET) || defined(BACDL_MSTP) || \
+  defined(BACDL_ETHERNET)  || defined(BACDL_BIP) 
+  /* valid! */
+#else
+  #error BACDL_ARCNET, BACDL_MSTP, BACDL_ETHERNET, or BACDL_BIP must be defined! 
+#endif
 
 /* returns number of bytes sent on success, negative on failure */
 int datalink_send_pdu(