In People v. Panizzon (1996) 13 Cal. 4th 68 [51 Cal. Rptr. 2d 851, 913 P.2d 1061], the Supreme Court held that a challenge attacking an integral part of the plea is, in substance, "a challenge to the validity of the plea, which requires compliance with the probable cause certificate requirements of section 1237.5 and rule 31(d)." ( People v. Panizzon, supra, at p. 73.) The challenge in Panizzon focused on the term of imprisonment to which the defendant had agreed as part of his plea agreement. (Ibid.) People v. Arwood (1985) 165 Cal. App. 3d 167 [211 Cal. Rptr. 307], although decided a decade before the Panizzon decision, supports this result as well. In Arwood, "appellant entered a plea of nolo contendere to a forcible rape count, and admitted the truth of the prior felony conviction of assault with a deadly weapon involving personal use of the deadly weapon. In exchange for his plea, the prosecution agreed to dismiss the remaining charge and prior felony allegation, and to strike the upper term for the rape count so that appellant would receive no greater than its midterm of six years in addition to section 667's five-year enhancement for the admitted prior serious felony conviction." ( Arwood, supra, 165 Cal. App. 3d at p. 170.) Arwood was sentenced to an 11-year term of imprisonment in accordance with the plea agreement. (Ibid.) On appeal, Arwood sought to challenge "the imposition of the five-year enhancement, arguing that his prior conviction of assault with a deadly weapon is not a 'serious felony' within the meaning of section 667 and should therefore be stricken." ( People v. Arwood, supra, 165 Cal. App. 3d at p. 171.) Arwood contended that "his challenge goes only to the validity of the sentence imposed and that the provisions of section 1237.5 are inapplicable to him in accordance with rule 31(d) because the judicial act of sentencing occurred after entry of his nolo contendere plea." ( Arwood, supra, 165 Cal. App. 3d at p. 172, italics in original.) The appellate court rejected Arwood's argument, explaining that the "argument ignores the fact that imposition of the enhancement related back to his admission of the prior felony conviction, which occurred at the time he entered the plea." (Ibid.; see also People v. Perry (1984) 162 Cal. App. 3d 1147, 1151 [209 Cal. Rptr. 414] "an appeal which questions proceedings before appellant's admission of the use of a firearm must comply with section 1237.5".