diff --git a/bacnet-stack/config.h b/bacnet-stack/config.h
index 8f159f07..accedf88 100644
--- a/bacnet-stack/config.h
+++ b/bacnet-stack/config.h
@@ -1,4 +1,4 @@
-#ifdef CONFIG_H
+#ifndef CONFIG_H
 #define CONFIG_H
 
 // Max number of bytes in an APDU.