magento2: Error in pulling currency exchange rates from Currency Converter API

Preconditions (*)

Clean Magento 2.3.1 installation.

Steps to reproduce (*)

1. Set store default currency to CAD, and add CAD and USD as allowed currencies.
2. Navigate to Stores -> Currency Rates
3. Select Currency Converter API as Import Service and click Import.

Expected result (*)

Get the latest rates.

Actual result (*)

Got following error: Notice: Undefined index: CAD_USD in /Users/hailong/dev/ttechtattoo/ttechtattoo_m2/vendor/magento/magento2ce/app/code/Magento/Directory/Model/Currency/Import/CurrencyConverterApi.php on line 91