diff --git a/nimble/controller/src/ble_ll.c b/nimble/controller/src/ble_ll.c
index 8f2b8e593..6456b8179 100644
--- a/nimble/controller/src/ble_ll.c
+++ b/nimble/controller/src/ble_ll.c
@@ -1283,9 +1283,20 @@ ble_ll_pdu_max_tx_octets_get(uint32_t usecs, int phy_mode)
 
     header_tx_time = g_ble_ll_pdu_header_tx_time[phy_mode];
 
+    /*
+     * Current conn max tx time can be too short to even send a packet header
+     * and this can happen if we changed connection form uncoded to coded phy.
+     * However, the lower bound for conn max tx time (all of them) depends on
+     * current phy (uncoded/coded) but it always allows to send at least 27
+     * bytes of payload thus we alwyas return at least 27 from here.
+     *
+     * Reference:
+     * Core v5.0, Vol 6, Part B, section 4.5.10
+     * see connEffectiveMaxTxTime and connEffectiveMaxRxTime definitions
+     */
+
     if (usecs < header_tx_time) {
-        // XXX: this is obviously incorrect, what should we do?
-        return 0;
+        return 27;
     }
 
     usecs -= header_tx_time;
@@ -1306,7 +1317,8 @@ ble_ll_pdu_max_tx_octets_get(uint32_t usecs, int phy_mode)
         assert(0);
     }
 
-    return octets;
+    /* see comment at the beginning */
+    return max(27, octets);
 }
 
 /**