This is referencing the covenant of life (or works) with Adam.

God entered into a covenant of life with man, upon condition of perfect obedience; forbidding him to eat of the tree of the knowledge of good and evil, upon the pain of death. Notice that it never names the fruit, only calls it “the tree of the knowledge of good and evil.”

Emphasis on the moral and covenantal meaning of the act, not the biological kind of fruit.