diff --git a/src/AsyncTCP.cpp b/src/AsyncTCP.cpp
index 5259400..9071ead 100644
--- a/src/AsyncTCP.cpp
+++ b/src/AsyncTCP.cpp
@@ -78,9 +78,20 @@ typedef struct {
 
 static xQueueHandle _async_queue;
 static TaskHandle_t _async_service_task_handle = NULL;
+
+
+SemaphoreHandle_t _slots_lock;
 const int _number_of_closed_slots = CONFIG_LWIP_MAX_ACTIVE_TCP;
-static int _closed_index = 0;
 static int _closed_slots[_number_of_closed_slots];
+static int _closed_index = []() {
+    _slots_lock = xSemaphoreCreateBinary();
+    xSemaphoreGive(_slots_lock);
+    for (int i = 0; i < _number_of_closed_slots; ++ i) {
+        _closed_slots[i] = 1;
+    }
+    return 1;
+}();
+
 
 static inline bool _init_async_event_queue(){
     if(!_async_queue){
@@ -554,22 +565,16 @@ AsyncClient::AsyncClient(tcp_pcb* pcb)
     _pcb = pcb;
     _closed_slot = -1;
     if(_pcb){
-        _closed_slot = 0;
-        if (_closed_index == 0) {
-            _closed_index = 1;
-            for (int i = 0; i < _number_of_closed_slots; ++ i) {
-                _closed_slots[i] = 1;
-            }
-        } else {
-            int closed_slot_min_index = _closed_slots[0];
-            for (int i = 0; i < _number_of_closed_slots; ++ i) {
-                if (_closed_slots[i] <= closed_slot_min_index && _closed_slots[i] != 0) {
-                    closed_slot_min_index = _closed_slots[i];
-                    _closed_slot = i;
-                }
+        xSemaphoreTake(_slots_lock, portMAX_DELAY);
+        int closed_slot_min_index = 0;
+        for (int i = 0; i < _number_of_closed_slots; ++ i) {
+            if ((_closed_slot == -1 || _closed_slots[i] <= closed_slot_min_index) && _closed_slots[i] != 0) {
+                closed_slot_min_index = _closed_slots[i];
+                _closed_slot = i;
             }
         }
         _closed_slots[_closed_slot] = 0;
+        xSemaphoreGive(_slots_lock);
 
         _rx_last_packet = millis();
         tcp_arg(_pcb, this);