When i enable the notify at app site with the six digit key,the connection disconnected.

Question

HI ,ALL , 
 i have change the project for the purpose of enter a six digit key at cellphone side to enable the notification with the board(52382). 
 But when i input the six digit passkey at cellphone side ,the connection disconnected. 
 Now,i will describe my configure steps.(There four steps in my changed project) 
 Premise 
 1.The project in use is 
 nRF5_SDK_11.0.0-2.alpha_bc3f6a0\examples\ble_peripheral\ble_app_uart 
 2.The Board in use is NRF52382 , 
 3.The cellPhone in use is IPhone6plus and Iphone5s 
 4.The app in use is LightBlue 
 #The First Step: 
 i defined some macro which will use in my project 
 #define IO_CAPS BLE_GAP_IO_CAPS_DISPLAY_ONLY
#define BOND 0
#define OOB 0
#define MITM 1
 
 #The Second Step 
 i added the function to enable the security purpose. 
 void resp_pair_request()
{
 ble_gap_sec_params_t sec_params;
 uint32_t err_code;
 memset(&sec_params,0,sizeof(ble_gap_sec_params_t));

 sec_params.bond = BOND;
 sec_params.io_caps = IO_CAPS;
 sec_params.max_key_size = 16;
 sec_params.min_key_size = 7;
 sec_params.oob = BOND;
 sec_params.mitm = MITM;
 err_code=sd_ble_gap_sec_params_reply(m_conn_handle,BLE_GAP_SEC_STATUS_SUCCESS,&sec_params,NULL);
 APP_ERROR_CHECK(err_code);
}
 
 #The Third Step 
 i changed the Event in the function 'on_ble_evt' 
 case BLE_GAP_EVT_SEC_PARAMS_REQUEST:
 printf("receive pair request
");
 resp_pair_request();
 break;

case BLE_GAP_EVT_PASSKEY_DISPLAY:
 printf("show passkey: ")
 for ( int i = 0; i < 6; i++)
 { 
 printf("%c",p_ble_evt->evt.gap_evt.params.passkey_display.passkey[i]);
 }
 break;
 
 #The Last Step 
 I change the security level in the rx_char_add function. 
 BLE_GAP_CONN_SEC_MODE_SET_ENC_WITH_MITM(&cccd_md.write_perm);

// BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);