I've got a unit test that's acting funny. The test passes, but the code coverage report shows one particular line, which is a comment, to be - not covered, and this affects the code coverage statistics:

The definition for the class being tested looks as follows:
class User implements UserInterface, \Serializable
When I remove the , \Serializable part, the problem disappears, but then my code will break in other ways, which is not a solution.
Does anyone have an idea why this could be happening?
phpunit --version
PHPUnit 3.6.11 by Sebastian Bergmann.