It is. I signed the same thing
Here is the California labor code related to your question
The labor code itself:
"2870. (a) Any provision in an employment agreement which provides
that an employee shall assign, or offer to assign, any of his or her
rights in an invention to his or her employer shall not apply to an
invention that the employee developed entirely on his or her own time
without using the employer's equipment, supplies, facilities, or
trade secret information except for those inventions that either:
(1) Relate at the time of conception or reduction to practice of
the invention to the employer's business, or actual or demonstrably
anticipated research or development of the employer; or
(2) Result from any work performed by the employee for the
employer.
(b) To the extent a provision in an employment agreement purports
to require an employee to assign an invention otherwise excluded from
being required to be assigned under subdivision (a), the provision
is against the public policy of this state and is unenforceable."
So, if on your own time, at home, you do a side project for say, a new coffee machine, then probably your employer can't claim any rights, but if your side project is related to your employer's business (say a new video game), then things may get complicated