MAY, J.
In 1999, Blakemore agreed to plead guilty to sexual misconduct with a minor as a Class C felony. The court accepted the agreement, entered the conviction, and imposed a four-year sentence with two years suspended to probation. The plea agreement provided Blakemore would “comply with the statutory requirements of registering with local law enforcement as a sex offender.” (Ex. 1 at 7.) When Blakemore was released to probation on April 27, 2000, he signed an “Order of Probation for Sex Offenders,” which said, “You must register on Indiana’s Sex Offender Registry within 7 days and maintain registration throughout Probation.”3 (Id. at 2.)
When Blakemore was convicted in 1999, a person who had committed Class C felony sexual misconduct with a minor was not subject to Indiana’s registration requirement for sex offenders. In 2001, public law 238 added a registration requirement for persons convicted of Blakemore’s underlying offense.4 Ind. Code § 11-8-8-4.5.
After being released to probation, Blakemore twice was returned to the Department of Correction because he violated probation. He was finally discharged, with his sentence completed and no remaining probation requirements, in February 2005. On at least five occasions after that, Blakemore registered as a sex offender. In April of 2008 he was arrested for failure to register as a sex offender.
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The State does not explicitly argue the registration requirement may be imposed on Blakemore without subjecting him to an ex post facto law in violation of the constitution. Rather, it asserts, the Wallace Court’s ex post facto analysis “may be ignored” because “Blackmore agreed to follow the statutory guidelines for sex offender registration and must therefore obey the statutes.” (Appellee’s Br. at 12.) However, the “statutory guidelines for sex offender registration” to which the State asserts Blackmore “agreed” did not apply to Blakemore at the time of his conviction. We therefore decline the State’s invitation to ignore the Wallace analysis.
The State urges us to affirm on the grounds Blakemore waived his constitutional argument or abandoned any such claim when he entered into his plea agreement. “Blakemore did something Wallace did not do . . . Blakemore agreed, explicitly and pursuant to the terms of his plea agreement, to comply with whatever the Indiana Code required of him, in the way of sex offender registration.” (Id. at 10) (emphasis supplied).
We decline to hold Blakemore “agreed” to requirements the Code did not impose when he entered into that agreement. A plea agreement is contractual in nature, binding the defendant, the state, and the trial court. Valenzuela v. State, 898 N.E.2d 480, 482 (Ind. Ct. App. 2008), trans. denied 915 N.E.2d 984 (Ind. 2009). The prosecutor and the defendant are the contracting parties, and if the trial court accepts the plea agreement, it is bound by its terms. Id. We look to principles of contract law when construing plea agreements to determine what is reasonably due the defendant. Id. The primary goal of contract interpretation is to give effect to the parties’ intent. Id. It has long been established that we cannot enlarge the terms of the contract or read into it additional provisions, Thomas v. Troxel, 26 Ind. App. 322, 59 N.E. 683, 685 (1901), and we decline the State’s invitation to so enlarge Blakemore’s plea agreement by reading into it an agreement to predict any changes in the law the legislature might subsequently enact, and to comply with any such changes.
The State next asserts Blakemore waived the ex post facto argument he now asserts on appeal because he “failed to raise any constitutional concerns when pleading guilty.” (Br. of Appellee at 7.) As explained above, the “constitutional concern” now before us did not exist when Blakemore entered into his plea agreement. Rather, his plea agreement contained a clause that by its very language did not apply to Blakemore, and neither he nor his counsel could be expected to predict what amendments our legislature might make to the sex offender registration act. His failure to raise that non-existent “concern” does not now preclude his ex post facto challenge.
Blakemore, like Wallace, was subjected to an ex post facto law in violation of the Indiana Constitution. When he was convicted in 1999 of Class C felony sexual misconduct with a minor, no statute required him to register as a sex offender. Therefore, application of the current version of the Sex Offender Registration Act to Blakemore “imposes burdens that have the effect of adding punishment beyond that which could have been imposed when his crime was committed,” Wallace, 905 N.E.2d at 384, and is unconstitutional.
KIRSCH, J., and DARDEN, J., concur.