[1.6.4-beta] Using driver.switchTo().activeElement() returns WebDriverException: Unhandled endpoint

I am trying to check if a text field has focus in it or not by using driver.switchTo().activeElement().
This used to work fine before upgrading to v1.6.x but not in v1.6.4-beta.
Java client: 4.1.2
Real device: iOS 10.2

I am getting
org.openqa.selenium.WebDriverException: Unhandled endpoint: /session/32C1494A-3073-4F3C-A5C3-FE9BFBCB08BB/element/active – http://localhost:8100/ with parameters {
wildcards = (

Any thoughts please?