diff --git a/dump1090.h b/dump1090.h
index 43e6109..90700ce 100644
--- a/dump1090.h
+++ b/dump1090.h
@@ -196,6 +196,7 @@ typedef enum {
 
 typedef enum {
     COMMB_UNKNOWN,
+    COMMB_AMBIGUOUS,
     COMMB_EMPTY_RESPONSE,
     COMMB_DATALINK_CAPS,
     COMMB_GICB_CAPS,
diff --git a/mode_s.c b/mode_s.c
index 6b30470..3767167 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -1569,6 +1569,8 @@ static const char *commb_format_to_string(commb_format_t format) {
     switch (format) {
     case COMMB_EMPTY_RESPONSE:
         return "empty response";
+    case COMMB_AMBIGUOUS:
+        return "ambiguous format";
     case COMMB_DATALINK_CAPS:
         return "BDS1,0 Datalink capabilities";
     case COMMB_GICB_CAPS: