The latest version adds a non-backward compatible change to passing the base URI in the constructor. Documentation says you should pass: ``` sp = SparkPost('YOUR API KEY', 'https://api.eu.sparkpost.com') ``` which is also what we are using for over a year. But in 1.3.9 the constructor actually expects it to be: ``` SparkPost('YOUR API KEY', 'api.eu.sparkpost.com') ``` as the constructor adds the 'https://' by itself, without backward compatibility check to see if it might have been added already.