diff --git a/mode_s.c b/mode_s.c
index 4093cc3..5c35962 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -716,10 +716,18 @@ static void decodeESIdentAndCategory(struct modesMessage *mm)
     mm->callsign[6] = ais_charset[getbits(me, 45, 50)];
     mm->callsign[7] = ais_charset[getbits(me, 51, 56)];
     mm->callsign[8] = 0;
+    mm->callsign_valid = 1;
 
-    // A common failure mode seems to be to intermittently send
-    // all zeros. Catch that here.
-    mm->callsign_valid = (strcmp(mm->callsign, "@@@@@@@@") != 0);
+    // actually valid?
+    for (unsigned i = 0; i < 8; ++i) {
+        if (!(mm->callsign[i] >= 'A' && mm->callsign[i] <= 'Z') &&
+            !(mm->callsign[i] >= '0' && mm->callsign[i] <= '9') &&
+            mm->callsign[i] != ' ') {
+            // Bad callsign, ignore it
+            mm->callsign_valid = 0;
+            break;
+        }
+    }
 
     mm->category = ((0x0E - mm->metype) << 4) | mm->mesub;
     mm->category_valid = 1;