sd_ble_gatts_characteristic_add

Question

Hi.I had been confused with parameters of sd_ble_gatts_characteristic_add( ).If I set parameters
like that: 
 static uint8_t ast_notify_char;

..............................

{

 ..............................

 attr_md.read_perm = p_ast_init->notify_ast_attr_md.read_perm;
 attr_md.write_perm = p_ast_init->notify_ast_attr_md.write_perm;
 attr_md.vloc = BLE_GATTS_VLOC_USER;
 attr_md.rd_auth = 0;
 attr_md.wr_auth = 0;
 attr_md.vlen = 1;

 memset(&attr_char_value, 0, sizeof(attr_char_value));

 attr_char_value.p_uuid = &ble_uuid;
 attr_char_value.p_attr_md = &attr_md;
 attr_char_value.init_len = 1;
 attr_char_value.init_offs = 0;
 attr_char_value.max_len = 20;
 attr_char_value.p_value = &ast_notify_char;

 return sd_ble_gatts_characteristic_add(p_ast->service_handle,
 &char_md,
 &attr_char_value,
 &p_ast->notify_ast_handles);
}
 
 I found RAM overflow after I had called notify_character( ) for sending 19 bytes.But if I set parameters like that it won't happen. 
 static uint8_t ast_notify_char[20];

...................................

{
 ...............................

 attr_char_value.p_uuid = &ble_uuid;
 attr_char_value.p_attr_md = &attr_md;
 attr_char_value.init_len = 20;
 attr_char_value.init_offs = 0;
 attr_char_value.max_len = 20;
 attr_char_value.p_value = ast_notify_char;

 .........................
}

uint32_t	notify_character( uint16_t notify_len, uint8_t *notifyTemp )

{

	uint32_t err_code;
	if( ast_app_handle.conn_handle != BLE_CONN_HANDLE_INVALID )
 {
 ble_gatts_hvx_params_t notify_params;
 
 notify_params.handle = ast_app_handle.notify_ast_handles.value_handle;
 notify_params.offset = 0;
 notify_params.type = BLE_GATT_HVX_NOTIFICATION;
 notify_params.p_len = &notify_len;
 notify_params.p_data = notifyTemp;
 
 err_code = sd_ble_gatts_hvx( ast_app_handle.conn_handle, &notify_params );
 }
	else
 {
 err_code = NRF_ERROR_INVALID_STATE;
 }
 return err_code;

}