Under California law, a petition to commit a person as a sexually violent predator must be filed while the person is in lawful custody. (Welf. & Inst. Code, § 6601, subd. (a)(2); unless otherwise indicated, all further statutory references are to the Welfare and Institutions Code.) In 2000, the Legislature imposed a requirement on what is now the Board of Parole Hearings (Board) that “good cause” be shown before extending the custody period up to “45 days beyond the person’s scheduled release date . . .” (§ 6601.3, subd. (a)), thus extending the time for filing the petition. The statute did not define the term “good cause.” That term has generally been considered as referring to “a legally sufficient ground or reason for a certain action.” (Zorrero v. Unemployment Ins. Appeals Bd. (1975) 47 Cal.App.3d 434, 439 [120 Cal.Rptr. 855].) In this case, the “certain action” to be taken pertains to extending a person’s custody. Therefore, the mention of “good cause” in section 6601.3’s subdivision (a) requires a legally sufficient reason for extending custody.
At issue here is the Board’s 2007 regulation defining good cause. The Board’s definition of good cause turns on the existence of a qualifying conviction and “[s]ome evidence that the person is likely to engage in sexually violent predatory criminal behavior.” (Cal. Code Regs., tit. 15, § 2600.1, subd. (d)(2); see maj. opn., ante, at p. 844.) But that determination is wholly unrelated to why the Board needs to extend a person’s custody “beyond the person’s scheduled release date . . . .” (§ 6601.3, subd. (a).) Therefore, the regulation does not implement the statutory requirement that “good cause” be shown justifying the Board’s extension of a person’s custody. I thus agree with the majority that the regulation is invalid.
I disagree, however, with the majority’s further conclusion that petitioner David Lucas and defendant Christopher Sharkey are not entitled to relief *859because, in extending custody beyond the scheduled release date, the Board made a good faith mistake of law. (See § 6601, subd. (a)(2) [stating that unlawful custody is not a ground for dismissing a petition to commit a person as a sexually violent predator if such custody was “the result of a good faith mistake of fact or law”].) I explore that issue below.
The Legislature imposed the good cause requirement on the Board in 2000. (Stats. 2000, ch. 41, § 1, p. 129.) Before that time, the custody extension statute (former § 6601.3; Stats. 1996, ch. 4, § 2, p. 16), enacted in 1996, lacked such a requirement. The Board’s then existing regulation, adopted before the 1996 statute, mentioned that custody extensions had to be based on “probable cause,” which the regulation defined as “some evidence” that the person met the sexually violent predator criteria set forth in the statutory scheme. According to the majority, the Board could in good faith have concluded that the Legislature’s 1996 enactment, which made no reference to either probable cause or good cause, ratified the Board’s preexisting “probable cause” regulation. (Maj. opn., ante, at pp. 856-857.) For reasons discussed below, I do not share that view.
The Board’s pre-1996 “probable cause” regulation suffers from the same defect as the Board’s post-2007 “good cause” regulation. Neither is based on reasons why the Board needs to extend custody. (The record reveals that defendant Sharkey’s custody was extended because a particular part-time employee of the Board had been laid off. As to petitioner Lucas, the record has no explanation for the extension of custody.)
In either instance, the Board defines probable cause and good cause as “some evidence” that the person is likely to engage in sexually violent predatory behavior. That definition is substantially similar to the test the Department of Corrections and Rehabilitation applies at the first stage in the process that leads to having a person in lawful custody committed as a sexually violent predator. That stage requires a determination by the department that a person “may be a sexually violent predator,” a determination that must be made “at least six months” before a person’s scheduled release date. (§ 6601, subd. (a)(1).) Here, the Board could not in good faith have mistakenly concluded that the “good cause” requirement of section 6601.3’s subdivision (a) contemplated only that the Board needed to make the same finding that the department has already made months earlier. To ascribe such *860a good faith belief to the Board, as the majority does, would mean the existence of good cause to extend a person’s custody in every case.
I would reverse the judgment of the Court of Appeal in each of these two consolidated cases.