The opinion of the court was delivered by: Justice Garman
 Docket No. 95574-Agenda 18-September 2003.
 Defendant, Tiffany Hopp, was found guilty by a jury of conspiracy to commit first degree murder. The appellate court reversed her conviction on the ground that the trial court committed plain error when it failed to give sua sponte a required jury instruction defining first degree murder. 336 Ill. App. 3d 523. We allowed the State's petition for leave to appeal. 177 Ill. 2d R. 315. We now reverse and affirm defendant's conviction.
 In the early morning of December 20, 2000, 16-year-old J.S. entered the home of James Hopp in Nauvoo and hit James on the head with a fireplace poker while he slept. James was seriously injured by the blow but has since recovered. J.S. and another juvenile, M.H., were arrested near the Hopp residence by a police officer who was responding to the scene. At the time of the attack, defendant Tiffany Hopp, James' estranged wife, was in California with the couple's four children, visiting Tiffany's mother. After defendant returned from California, she went to the Hancock County sheriff's department to be interviewed about the attack on her husband. She gave two statements regarding her involvement in the attack, the second of which was videotaped. She was then arrested and later charged by information with conspiracy to commit first degree murder, and with attempted first degree murder and home invasion on a theory of accountability for the actions of J.S. and M.H.
 J.S. and M.H. testified against defendant at trial. Both testified that J.S. was defendant's boyfriend. J.S. was 16 years old at the time of the attack on James, while defendant was 26. Both J.S. and M.H. testified about numerous conversations they had with defendant in the weeks prior to the attack. During those conversations the three discussed various plans to kill James, including using a shotgun, burning down the house, and cutting James' throat. Defendant criticized these plans. For example, she opined that a shotgun would make too much noise. The trio also discussed fabricating alibis by, for example, going to a movie theater in another town, obtaining tickets, and then slipping out of the theater to go to Nauvoo and kill James.
 J.S. testified that "[w]e finalized the plan" shortly before defendant and the children left for California. The final plan was for J.S. and M.H. to go to the Hopp house early in the morning during the weekend. J.S. testified that it was decided to use the poker at his suggestion, and that he had the idea because he "saw it on TV a lot." M.H. testified that his role was to accompany J.S. to make sure he would go through with it. Both testified that M.H. was to receive a car for his involvement while defendant would get custody of the four children, the house, and possibly proceeds from insurance on James' life.
 The State played defendant's videotaped statement for the jury. In it she maintained that she discussed killing James with J.S. and M.H. only so she could talk the boys out of it by pointing out flaws in their plans. She also claimed she was afraid J.S. would leave her if she told him not to kill James. Defendant also introduced a letter that J.S. wrote to James after the attack in which J.S. stated that defendant "didn't plan anything, she didn't talk me into anything, if anyone deserves her charges it's [M.H.]."
 A police officer who was present when defendant gave both statements testified that defendant was generally more "accountable" in the untaped statement. The officer testified that in the untaped statement defendant admitted that when she sent a message to J.S. via the internet stating that he had one more day, she meant that he had one more day left in which to kill James. In the taped statement she claimed that she meant J.S. had one more day to be without her, but admitted she did not correct him when she realized he understood it to mean that he had one more day to kill James. A second officer testified that while he was transporting defendant to court the day after her arrest she asked him why she was charged with solicitation to commit murder. Before he could answer, defendant stated, "We committed a conspiracy to kill [James] but we didn't do a solicitation to kill [James]." J.S. testified that when he wrote to James that defendant did not plan anything, the statement was not accurate. J.S. testified further that defendant did not come up with any of the ideas about how to kill James.
 The jury was instructed on the elements of conspiracy and the other two charged crimes. However, neither party tendered a jury instruction defining first degree murder, the object offense of the conspiracy, and no such instruction was given. The failure to instruct was not raised in a posttrial motion. The jury found defendant guilty of conspiracy to commit first degree murder and not guilty of attempted first degree murder and home invasion. The court later sentenced defendant to seven years in the Department of Corrections.
 Defendant did not tender a jury instruction defining first degree murder, nor did she assert in her posttrial motion that it was error to omit the instruction. On appeal, defendant argued that the court erroneously failed to give a nonpattern jury instruction she had tendered and that the court committed plain error when it failed to give an instruction defining first degree murder sua sponte. The appellate court held that the court did not err in refusing the nonpattern jury instruction but reversed defendant's conviction, over a dissent, on the ground that the failure to instruct the jury on the definition of first degree murder for purposes of the conspiracy count was plain error. The sole issue before us is whether that failure to instruct on the definition of first degree murder was plain error.
 Supreme Court Rule 451(a) requires that in a criminal case, if the court determines the jury should be instructed on a subject, and the Illinois Pattern Jury Instruction (IPI), Criminal, contains an applicable instruction, then the IPI instruction "shall" be given unless the court determines it does not accurately state the law. 177 Ill. 2d R. 451(a); People v. Novak, 163 Ill. 2d 93, 116 (1994). The court in this case gave the IPI instructions for a charge of conspiracy other than certain drug conspiracies. See Illinois Pattern Jury Instructions, Criminal, Nos. 6.03, 6.04 (4th ed. 2000) (hereinafter IPI Criminal 4th). The committee note to Instruction 6.03 states that the court "must" also give an instruction defining the offense that is the alleged subject of the conspiracy. IPI Criminal 4th No. 6.03, Committee Note. The user's guide to IPI Criminal 4th states, "If a Committee Note indicates to give another instruction, that is a mandatory requirement." IPI Criminal 4th, User's Guide, at VIII. Thus it was error for the court to fail to give the IPI instruction defining first degree murder.
 Supreme Court Rule 366 provides that a party that fails to tender a jury instruction may not raise the failure to give the instruction on appeal. 155 Ill. 2d R. 366(b)(2)(i). However, Rule 451(c) provides that "substantial defects [in jury instructions in criminal cases] are not waived by failure to make timely objections thereto if the interests of justice require." 177 Ill. 2d R. 451(c).
 Rule 451(c)'s exception to the waiver rule for substantial defects applies when there is a grave error or when the case is so factually close that fundamental fairness requires that the jury be properly instructed. People v. Thurman, 104 Ill. 2d 326, 329-30 (1984); People v. Huckstead, 91 Ill. 2d 536, 544 (1982). The tests for application of Rule 451(c)'s exception to the waiver rule are "strict tests" that "demonstrate that the exception to the waiver rule is limited and is applicable only to serious errors which severely threaten the fundamental fairness of the defendant's trial." People v. Roberts, 75 Ill. 2d 1, 15 (1979). The function of jury instructions is to convey to the jurors the law that applies to the facts so they can reach a correct conclusion. People v. Fuller, 205 Ill. 2d 308, 343 (2002). Thus, the erroneous omission of a jury instruction rises to the level of plain error only when the omission creates a serious risk that the jurors incorrectly convicted the defendant because they did not understand the applicable law, so as to severely threaten the fairness of the trial. In People v. Ogunsola, we reversed defendant's conviction because an omitted jury instruction "removed from the jury's consideration a disputed issue essential to the determination of defendant's guilt or innocence." People v. Ogunsola, 87 Ill. 2d 216, 223 (1981). The defendant in Ogunsola was charged with deceptive practices. The jury was given an IPI instruction that it should find the defendant guilty if the State proved beyond a reasonable doubt that the defendant wrote a check with the intent to pay the victim while knowing the check would not be paid. The jury should have been instructed that it also had to find the defendant intended to defraud the victim, because intent to defraud is an element of the offense of deceptive practices. We reasoned that ...