diff --git a/mode_s.c b/mode_s.c
index 606ddf7..e8387a1 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -681,12 +681,12 @@ int decodeModesMessage(struct modesMessage *mm, unsigned char *msg)
             mm->airground = AG_UNCERTAIN;
     }
 
-    if (!mm->correctedbits && (mm->msgtype == 17 || mm->msgtype == 18 || (mm->msgtype == 11 && mm->IID == 0))) {
-        // No CRC errors seen, and either it was an DF17/18 extended squitter
+    if (!mm->correctedbits && (mm->msgtype == 17 || (mm->msgtype == 11 && mm->IID == 0))) {
+        // No CRC errors seen, and either it was an DF17 extended squitter
         // or a DF11 acquisition squitter with II = 0. We probably have the right address.
 
-        // We wait until here to do this as we may have needed to decode an ES to note
-        // the type of address in DF18 messages.
+        // Don't do this for DF18, as a DF18 transmitter doesn't necessarily have a
+        // Mode S transponder.
 
         // NB this is the only place that adds addresses!
         icaoFilterAdd(mm->addr);
diff --git a/net_io.c b/net_io.c
index 5148b6a..3bc2f85 100644
--- a/net_io.c
+++ b/net_io.c
@@ -1380,7 +1380,7 @@ char *generateAircraftJson(const char *url_path, int *len) {
                       10 * log10((a->signalLevel[0] + a->signalLevel[1] + a->signalLevel[2] + a->signalLevel[3] +
                                   a->signalLevel[4] + a->signalLevel[5] + a->signalLevel[6] + a->signalLevel[7] + 1e-5) / 8));
 
-        if (p >= end) {
+        if ((p + 10) >= end) { // +10 to leave some space for the final line
             // overran the buffer
             int used = line_start - buf;
             buflen *= 2;