California Code, Civil Code - CIV § 3339.10

(b)(1) In proceedings or discovery undertaken in a civil action to enforce Chapter 2 (commencing with Section 1940) of Title 5 of Part 4 of Division 3, Chapter 2 (commencing with Section 789) of Title 2 of Part 2 of Division 2 of this code, or under Chapter 4 (commencing with Section 1159) of Title 3 of Part 3 of the Code of Civil Procedure, or in any civil action involving a tenant's housing rights, no inquiry shall be permitted into a person's immigration or citizenship status, except as follows:

(2) The assertion of an affirmative defense to an unlawful detainer action under Section 1161.4 of the Code of Civil Procedure does not constitute cause under this subdivision for discovery or other inquiry into that person's immigration or citizenship status.