Petitioner Williams entered into a plea bargain in which he pleaded guilty to an escape in return for a specified sentence which included presentence credits. After sentencing, the Department of Corrections informed the court that he was not entitled to the credits, because he was serving a prison term during the presentence time period. Without holding a hearing, the trial court struck the presentence credit provision. Williams’ petition for writ of habeas corpus was granted. Because petitioner was not entitled to presentence credit while serving a prison term for another offense, awarding presentence credit made the sentence unauthorized. However, the striking of the credits violated appellant’s plea bargain. As the bargain could not be carried out according to its terms, it had to be vacated. The presentence credit was a material term of the plea bargain entitling the petitioner to withdraw his plea.