diff --git a/dump1090.c b/dump1090.c
index 0039085..8c0ebc6 100644
--- a/dump1090.c
+++ b/dump1090.c
@@ -713,6 +713,7 @@ void backgroundTasks(void) {
 
     if ((Modes.json_dir || Modes.net_http_port) && now >= next_history) {
         char filebuf[PATH_MAX];
+        int rewrite_receiver_json = (Modes.json_aircraft_history[HISTORY_SIZE-1].content == NULL);
 
         free(Modes.json_aircraft_history[Modes.json_aircraft_history_next].content); // might be NULL, that's OK.
         Modes.json_aircraft_history[Modes.json_aircraft_history_next].content =
@@ -722,6 +723,10 @@ void backgroundTasks(void) {
         writeJsonToFile(filebuf, generateHistoryJson);
 
         Modes.json_aircraft_history_next = (Modes.json_aircraft_history_next+1) % HISTORY_SIZE;
+
+        if (rewrite_receiver_json)
+            writeJsonToFile("receiver.json", generateReceiverJson); // number of history entries changed
+
         next_history = now + HISTORY_INTERVAL;
     }
 }
@@ -977,7 +982,7 @@ int main(int argc, char **argv) {
     }
     if (Modes.net) modesInitNet();
 
-    writeJsonToFile("receiver.json", generateReceiverJson); // once only on startup
+    writeJsonToFile("receiver.json", generateReceiverJson); // once on startup
 
     // If the user specifies --net-only, just run in order to serve network
     // clients without reading data from the RTL device
diff --git a/net_io.c b/net_io.c
index f9bbc0f..d0719fe 100644
--- a/net_io.c
+++ b/net_io.c
@@ -737,14 +737,21 @@ char *generateAircraftJson(const char *url_path, int *len) {
 char *generateReceiverJson(const char *url_path, int *len)
 {
     char *buf = (char *) malloc(1024), *p = buf;
+    int history_size;
 
     (void)url_path;  // unused
 
+    // work out number of valid history entries
+    if (Modes.json_aircraft_history[HISTORY_SIZE-1].content == NULL)
+        history_size = Modes.json_aircraft_history_next;
+    else
+        history_size = HISTORY_SIZE;
+
     p += sprintf(p, "{ " \
                  "\"version\" : \"%s\", "
                  "\"refresh\" : %d, "
                  "\"history\" : %d",
-                 MODES_DUMP1090_VERSION, Modes.json_interval * 1000, HISTORY_SIZE);
+                 MODES_DUMP1090_VERSION, Modes.json_interval * 1000, history_size);
 
     if (Modes.json_location_accuracy && (Modes.fUserLat != 0.0 || Modes.fUserLon != 0.0)) {
         if (Modes.json_location_accuracy == 1) {