diff --git a/extmod/btstack/modbluetooth_btstack.c b/extmod/btstack/modbluetooth_btstack.c
index 243b6e38ca..8ce2db74ec 100644
--- a/extmod/btstack/modbluetooth_btstack.c
+++ b/extmod/btstack/modbluetooth_btstack.c
@@ -166,7 +166,7 @@ STATIC void btstack_remove_pending_operation(mp_btstack_pending_op_t *pending_op
 
 // Register a pending background operation -- copies the buffer, and makes it known to the GC.
 STATIC mp_btstack_pending_op_t *btstack_enqueue_pending_operation(uint16_t op_type, uint16_t conn_handle, uint16_t value_handle, const uint8_t *buf, size_t len) {
-    DEBUG_printf("btstack_enqueue_pending_operation op_type=%d conn_handle=%d value_handle=%d len=%zu\n", op_type, conn_handle, value_handle, len);
+    DEBUG_printf("btstack_enqueue_pending_operation op_type=%d conn_handle=%d value_handle=%d len=%lu\n", op_type, conn_handle, value_handle, len);
     mp_btstack_pending_op_t *pending_op = m_new_obj_var(mp_btstack_pending_op_t, uint8_t, len);
     pending_op->op_type = op_type;
     pending_op->conn_handle = conn_handle;
@@ -1079,8 +1079,44 @@ int mp_bluetooth_gatts_write(uint16_t value_handle, const uint8_t *value, size_t
     return mp_bluetooth_gatts_db_write(MP_STATE_PORT(bluetooth_btstack_root_pointers)->gatts_db, value_handle, value, value_len);
 }
 
+#if !MICROPY_TRACKED_ALLOC
+#error "btstack requires MICROPY_TRACKED_ALLOC"
+#endif
+
+typedef struct {
+    btstack_context_callback_registration_t btstack_registration;
+    int gatts_op;
+    uint16_t conn_handle;
+    uint16_t value_handle;
+    size_t value_len;
+    uint8_t value[];
+} notify_indicate_pending_op_t;
+
+// Called in response to a gatts_notify/indicate being unable to complete, which then calls
+// att_server_request_to_send_notification.
+STATIC void btstack_notify_indicate_ready_handler(void *context) {
+    MICROPY_PY_BLUETOOTH_ENTER
+    notify_indicate_pending_op_t *pending_op = (notify_indicate_pending_op_t *)context;
+    DEBUG_printf("btstack_notify_indicate_ready_handler gatts_op=%d conn_handle=%d value_handle=%d len=%lu\n", pending_op->gatts_op, pending_op->conn_handle, pending_op->value_handle, pending_op->value_len);
+    int err = ERROR_CODE_SUCCESS;
+    switch (pending_op->gatts_op) {
+        case MP_BLUETOOTH_GATTS_OP_NOTIFY:
+            err = att_server_notify(pending_op->conn_handle, pending_op->value_handle, pending_op->value, pending_op->value_len);
+            DEBUG_printf("btstack_notify_indicate_ready_handler: sending notification err=%d\n", err);
+            break;
+        case MP_BLUETOOTH_GATTS_OP_INDICATE:
+            err = att_server_indicate(pending_op->conn_handle, pending_op->value_handle, pending_op->value, pending_op->value_len);
+            DEBUG_printf("btstack_notify_indicate_ready_handler: sending indication err=%d\n", err);
+            break;
+    }
+    assert(err == ERROR_CODE_SUCCESS);
+    (void)err;
+    MICROPY_PY_BLUETOOTH_EXIT
+    m_tracked_free(pending_op);
+}
+
 int mp_bluetooth_gatts_notify_indicate(uint16_t conn_handle, uint16_t value_handle, int gatts_op, const uint8_t *value, size_t value_len) {
-    DEBUG_printf("mp_bluetooth_gatts_notify_indicate\n");
+    DEBUG_printf("mp_bluetooth_gatts_notify_indicate: gatts_op=%d\n", gatts_op);
 
     if (!mp_bluetooth_is_active()) {
         return ERRNO_BLUETOOTH_NOT_ACTIVE;
@@ -1107,10 +1143,33 @@ int mp_bluetooth_gatts_notify_indicate(uint16_t conn_handle, uint16_t value_hand
     }
     MICROPY_PY_BLUETOOTH_EXIT
 
-    if (err == BTSTACK_ACL_BUFFERS_FULL) {
-        DEBUG_printf("mp_bluetooth_gatts_notify_indicate: ACL buffer full, scheduling callback\n");
+    if (err == BTSTACK_ACL_BUFFERS_FULL || err == ATT_HANDLE_VALUE_INDICATION_IN_PROGRESS) {
+        DEBUG_printf("mp_bluetooth_gatts_notify_indicate: ACL buffer full / indication in progress, scheduling callback\n");
 
-        // TODO: re-implement the handling for this.
+        // Copy the value and ask btstack to let us know when it can be sent.
+        notify_indicate_pending_op_t *pending_op = m_tracked_calloc(1, sizeof(notify_indicate_pending_op_t) + value_len);
+        pending_op->btstack_registration.context = pending_op;
+        pending_op->btstack_registration.callback = &btstack_notify_indicate_ready_handler;
+        pending_op->gatts_op = gatts_op;
+        pending_op->conn_handle = conn_handle;
+        pending_op->value_handle = value_handle;
+        pending_op->value_len = value_len;
+        memcpy(pending_op->value, value, value_len);
+
+        MICROPY_PY_BLUETOOTH_ENTER
+        switch (gatts_op) {
+            case MP_BLUETOOTH_GATTS_OP_NOTIFY:
+                err = att_server_request_to_send_notification(&pending_op->btstack_registration, conn_handle);
+                break;
+            case MP_BLUETOOTH_GATTS_OP_INDICATE:
+                err = att_server_request_to_send_indication(&pending_op->btstack_registration, conn_handle);
+                break;
+        }
+        MICROPY_PY_BLUETOOTH_EXIT
+
+        if (err != ERROR_CODE_SUCCESS) {
+            m_tracked_free(pending_op);
+        }
     }
 
     return btstack_error_to_errno(err);
diff --git a/ports/rp2/mpconfigport.h b/ports/rp2/mpconfigport.h
index d862812f9b..da4548a473 100644
--- a/ports/rp2/mpconfigport.h
+++ b/ports/rp2/mpconfigport.h
@@ -74,7 +74,7 @@
 #define MICROPY_OPT_COMPUTED_GOTO               (1)
 
 // Python internal features
-#define MICROPY_TRACKED_ALLOC                   (MICROPY_SSL_MBEDTLS)
+#define MICROPY_TRACKED_ALLOC                   (MICROPY_SSL_MBEDTLS || MICROPY_BLUETOOTH_BTSTACK)
 #define MICROPY_READER_VFS                      (1)
 #define MICROPY_ENABLE_GC                       (1)
 #define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF  (1)
diff --git a/ports/stm32/mpconfigport.h b/ports/stm32/mpconfigport.h
index ee12ab8140..e3283c8f57 100644
--- a/ports/stm32/mpconfigport.h
+++ b/ports/stm32/mpconfigport.h
@@ -65,7 +65,7 @@
 #endif
 
 // Python internal features
-#define MICROPY_TRACKED_ALLOC       (MICROPY_SSL_MBEDTLS)
+#define MICROPY_TRACKED_ALLOC       (MICROPY_SSL_MBEDTLS || MICROPY_BLUETOOTH_BTSTACK)
 #define MICROPY_READER_VFS          (1)
 #define MICROPY_ENABLE_GC           (1)
 #define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF (1)
diff --git a/ports/unix/mpconfigport.h b/ports/unix/mpconfigport.h
index a0b9192bfc..8eafb6119b 100644
--- a/ports/unix/mpconfigport.h
+++ b/ports/unix/mpconfigport.h
@@ -117,6 +117,9 @@ typedef long mp_off_t;
 #define MICROPY_HELPER_LEXER_UNIX   (1)
 #define MICROPY_VFS_POSIX           (1)
 #define MICROPY_READER_POSIX        (1)
+#ifndef MICROPY_TRACKED_ALLOC
+#define MICROPY_TRACKED_ALLOC       (MICROPY_BLUETOOTH_BTSTACK)
+#endif
 
 // VFS stat functions should return time values relative to 1970/1/1
 #define MICROPY_EPOCH_IS_1970       (1)
diff --git a/ports/unix/variants/coverage/mpconfigvariant.h b/ports/unix/variants/coverage/mpconfigvariant.h
index 6107a4a556..2a48e57e7f 100644
--- a/ports/unix/variants/coverage/mpconfigvariant.h
+++ b/ports/unix/variants/coverage/mpconfigvariant.h
@@ -39,6 +39,6 @@
 
 // Enable additional features.
 #define MICROPY_DEBUG_PARSE_RULE_NAME  (1)
-#define MICROPY_TRACKED_ALLOC           (1)
+#define MICROPY_TRACKED_ALLOC          (1)
 #define MICROPY_WARNINGS_CATEGORY      (1)
 #define MICROPY_PY_UCRYPTOLIB_CTR      (1)
diff --git a/py/gc.c b/py/gc.c
index ba5c569d50..ad3e110407 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -746,7 +746,9 @@ void *gc_alloc_with_finaliser(mp_uint_t n_bytes) {
 // TODO: freeing here does not call finaliser
 void gc_free(void *ptr) {
     if (MP_STATE_THREAD(gc_lock_depth) > 0) {
-        // TODO how to deal with this error?
+        // Cannot free while the GC is locked. However free is an optimisation
+        // to reclaim the memory immediately, this means it will now be left
+        // until the next collection.
         return;
     }