Is a waiver of the Miranda rights invalid or involuntary?

The relevant law is not in dispute. Under the Fourteenth Amendment of the U.S. Constitution and article I, section 15 of the California Constitution, a defendant's waiver of Miranda rights is invalid and any subsequent confession inadmissible if not voluntarily given. (People v. Massie (1998) 19 Cal.4th 550, 576.) "A statement is involuntary if it is 'not " 'the product of a rational intellect and a free will.' " ' [Citation.]" (People v. Rundle (2008) 43 Cal.4th 76, 114, overruled on another ground in People v. Doolin (2009) 45 Cal.4th 390, 421, fn. 22.)