diff --git a/src/e1_input.c b/src/e1_input.c
index 7d89c7f..153a413 100644
--- a/src/e1_input.c
+++ b/src/e1_input.c
@@ -35,8 +35,10 @@
 
 //#define AF_COMPATIBILITY_FUNC
 //#include <compat_af_isdn.h>
+#ifndef AF_ISDN
 #define AF_ISDN 34
 #define PF_ISDN AF_ISDN
+#endif
 
 #include <openbsc/select.h>
 #include <openbsc/msgb.h>