diff --git a/features/FEATURE_BLE/source/generic/GenericGap.cpp b/features/FEATURE_BLE/source/generic/GenericGap.cpp
index e21bda6fbc..a6be0dc6ff 100644
--- a/features/FEATURE_BLE/source/generic/GenericGap.cpp
+++ b/features/FEATURE_BLE/source/generic/GenericGap.cpp
@@ -64,6 +64,10 @@ static const GapScanningParams default_scan_params;
 static const mbed_error_status_t mixed_scan_api_error =
     MBED_MAKE_ERROR(MBED_MODULE_BLE, MBED_ERROR_CODE_BLE_USE_INCOMPATIBLE_API);
 
+static const mbed_error_status_t illegal_state_error =
+    MBED_MAKE_ERROR(MBED_MODULE_BLE, MBED_ERROR_CODE_BLE_ILLEGAL_STATE);
+
+
 /*
  * Return true if value is included in the range [lower_bound : higher_bound]
  */
@@ -1792,6 +1796,8 @@ void GenericGap::on_connection_parameter_request(const pal::GapRemoteConnectionP
                     supervision_timeout_t(e.supervision_timeout)
                 )
             );
+        } else {
+            MBED_ERROR(illegal_state_error, "Event handler required if connection params are user handled");
         }
     } else {
         _pal_gap.accept_connection_parameter_request(
diff --git a/platform/mbed_error.h b/platform/mbed_error.h
index 692336154e..4301f2ff67 100644
--- a/platform/mbed_error.h
+++ b/platform/mbed_error.h
@@ -792,6 +792,7 @@ typedef enum _mbed_error_code {
     MBED_DEFINE_SYSTEM_ERROR(AUTHENTICATION_FAILED, 69),                /* 325      Authentication Failed */
     MBED_DEFINE_SYSTEM_ERROR(RBP_AUTHENTICATION_FAILED, 70),            /* 326      Rollback Protection Authentication Failed */
     MBED_DEFINE_SYSTEM_ERROR(BLE_USE_INCOMPATIBLE_API, 71),             /* 327      Concurrent use of incompatible versions of a BLE API */
+    MBED_DEFINE_SYSTEM_ERROR(BLE_ILLEGAL_STATE, 72),                    /* 328      BLE stack entered illegal state */
 
     //Everytime you add a new system error code, you must update
     //Error documentation under Handbook to capture the info on