What is the test for corroboration under section 1111 of the Penal Code?

Penal Code section 1111 provides in pertinent part: "A conviction can not be had upon the testimony of an accomplice unless it [is] corroborated by such other evidence as shall tend to connect the defendant with the commission of the offense; and the corroboration is not sufficient if it merely shows the commission of the offense or the circumstances thereof." ( 1111.) " '[W]hen there is sufficient evidence that a witness is an accomplice, the trial court is required on its own motion to instruct the jury on the principles governing the law of accomplices,' including the need for corroboration. [Citation.]" (People v. Tobias (2001) 25 Cal.4th 327, 331.)