diff --git a/openbsc/include/openbsc/signal.h b/openbsc/include/openbsc/signal.h
index 4a583f6..c065f90 100644
--- a/openbsc/include/openbsc/signal.h
+++ b/openbsc/include/openbsc/signal.h
@@ -54,6 +54,7 @@
 enum signal_nm {
 	S_NM_SW_ACTIV_REP,	/* GSM 12.21 software activated report */
 	S_NM_FAIL_REP,		/* GSM 12.21 failure event report */
+	S_NM_NACK,		/* GSM 12.21 various NM_MT_*_NACK happened */
 };
 
 /* SS_LCHAN signals */