**Symfony version(s) affected**: 4.1 **Description** #26702 introduces an annoying BC break: when using Prophecy, it's not possible to prohesize our interfaces anymore. **How to reproduce** $serializerException = $this->prophesize(ExceptionInterface::class); $serializerException->willExtend(\Exception::class); $serializerException->reveal(); Was working before, now throws `Prophecy\Exception\Doubler\MethodNotExtendableException: Method `getMessage` is not extendable, so can not be added.` **Possible Solution** Reverting #26702. (I'm trying to figure another solution).