I Just had the same kind of error by using an assert for an entity :

     * @Assert\Email(
     *     message = "The email '{{ value }}' is not a valid email.",
     *     mode = 'strict',
     *     normalizer = 'trim'
     * )

Turning it into

     * @Assert\Email(
     *     message = "The email '{{ value }}' is not a valid email.",
     *     mode = "strict",
     *     normalizer = "trim"
     * )