
252 S.E.2d 270 (1979)
40 N.C. App. 188
Everett A. LUDWIG and wife, Barbara A. Ludwig
v.
Malcolm S. HART and wife, Doris Ann Hart.
No. 781SC363.
Court of Appeals of North Carolina.
March 6, 1979.
*272 White, Hall, Mullen, Brumsey & Small by Gerald F. White and John H. Hall, Jr., Elizabeth City, for plaintiffs-appellees.
Leroy, Wells, Shaw, Hornthal, Riley & Shearin, P. A. by John G. Gaw, Jr., Kitty Hawk, for defendants-appellants.
MITCHELL, Judge.
The defendants first contend that the failure of the plaintiffs to join the trustee in the deed of trust as a party to this action renders the judgment of foreclosure void. G.S. 1A-1, Rule 19(a) requires that a person must be joined as a party to an action if that person is "united in interest" with another party to the action. A person is "united in interest" with another party when that person's presence is necessary in order for the court to determine the claim before it without prejudicing the rights of a party before it or the rights of others not before the court. In the present case, the trial court's determination of the plaintiffs' claim resulted in the divestment of the trustee's legal title to the property held under the deed of trust. The trustee had both a right and an obligation to present evidence of any defenses he might have to that divestment. As the trustee was not before the court, however, his right to present evidence of possible defenses to the plaintiffs' action was prejudiced by the court's determination of their claim. Since the trial court could not determine the claim before it without prejudicing the rights of the trustee, it was required, even in the absence of a motion by one of the parties, to order the trustee summoned to appear in the action. G.S. 1A-1, Rule 19(b). A judgment which is determinative of a claim arising in an action to which one who is "united in interest" with one of the parties has not been joined is void. Therefore, in this action to which the trustee in the deed of trust was not made a party, that portion of the judgment directing foreclosure and the sale of the property described in the deed of trust is void.
The defendants next contend that the trial court erred in granting the plaintiffs' motion for a directed verdict with regard to the defendants' counterclaim. A motion for a directed verdict requires the trial court determine whether the evidence presented at trial is sufficient as a matter of law to support the nonmoving party's *273 claim against the moving party. Sibbett v. Lifestock, Inc., 37 N.C.App. 704, 247 S.E.2d 2 (1978). In determining whether the evidence presented is sufficient to withstand a motion for a directed verdict, the trial court must consider all of the evidence in the light most favorable to the nonmoving party giving that party the benefit of every reasonable inference to be drawn therefrom. Byerly v. Byerly, 38 N.C.App. 551, 248 S.E.2d 433 (1978). If the evidence tends to establish the claim of the nonmoving party when considered in this light, the trial court commits reversible error by granting the motion.
The defendants' counterclaim in the present case alleged among other things that the contract to purchase the business, the note, and the deed of trust were each unenforceable as the defendant Malcolm S. Hart did not have sufficient mental capacity to enter into a contract. A person has sufficient mental capacity to enter a contract if he is possessed of
the ability to understand the nature of the act in which he is engaged and its scope and effect, or its nature and consequences, not that he should be able to act wisely or discreetly, nor to drive a good bargain, but that he should be in such possession of his faculties as to enable him to know at least what he is doing and to contract understandingly. There is no particular formula to be used in such cases . . . but the law in this respect should be explained to the jury with reference to the special and peculiar facts of the case being tried, and under the guidance of such general principles as have been settled and declared by the courts.
Sprinkle v. Wellborn, 140 N.C. 163, 181, 52 S.E. 666, 672 (1905). Accord, Goins v. McLoud, 231 N.C. 655, 58 S.E.2d 634 (1950); Cameron v. Power Co., 138 N.C. 365, 50 S.E. 695 (1905). Anyone may testify as to his or her opinion of the mental condition of another person if he or she has a reasonable basis upon which to form that opinion. State v. Brower, 289 N.C. 644, 224 S.E.2d 551 (1976); Moore v. Insurance Co., 266 N.C. 440, 146 S.E.2d 492 (1966); Clary v. Clary, 24 N.C. 78 (1841); Annot., 40 A.L. R.2d 15 (1955).
Dr. Franklin Stanford Burroughs, a licensed physician testified in the present case that he had observed Mr. Hart during September, October and November of 1974 and that he had an opinion satisfactory to himself as to whether Mr. Hart knew the nature and consequences of his acts. Dr. Burroughs indicated that during that period of time Mr. Hart fantasized and that such fantasizing was associated with all psychotic illnesses. He was of the opinion that Mr. Hart did not test reality well and that reality testing is that which allows people to conduct their affairs in a normal fashion. Additionally, Edgar M. Johnson, Jr., a certified public accountant, testified that, based upon his observations of and conversations with Mr. Hart, he was of the opinion that Mr. Hart did not have the mental capacity to understand the nature and consequences of his acts.
Such evidence, when considered in the light most favorable to the defendants, was sufficient to support their claim that the contracts in question were unenforceable due to the mental incapacity to contract of the defendant, Malcolm S. Hart, at the times in question. Therefore, that portion of the trial court's judgment granting the plaintiffs' motion for a directed verdict with regard to the defendants' counterclaim was erroneous and must be reversed.
Although not specifically assigned as error by the defendants, we have chosen as a matter of judicial efficiency to consider ex mero motu whether the trial court erred in granting a directed verdict in favor of the plaintiffs with regard to their original claim. The plaintiffs alleged in their complaint, and therefore had the burden of proving, that the defendants had defaulted on a note. By their answer, the defendants denied that they had defaulted on the note. At no time either before or during the trial did the defendants admit they had defaulted on the note.
The trial judge may not direct a verdict in favor of the party having the burden *274 of proof when his right to recover depends upon the credibility of his witnesses, even though the evidence is uncontradicted, the defendants' denial of an alleged fact, necessary to the plaintiff's right of recovery, being sufficient to raise an issue as to the existence of that fact, even though he offers no evidence tending to contradict that offered by the plaintiff. Cutts v. Casey, 278 N.C. 390, 417-422, 180 S.E.2d 297.
Rose v. Motor Sales, 288 N.C. 53, 61-62, 215 S.E.2d 573, 578 (1975). Therefore, the trial court erred in granting a directed verdict in favor of the plaintiffs on their original complaint and this portion of the judgment must be reversed.
We have reviewed the defendants' remaining assignments of error but find it unnecessary to discuss them as they are not likely to recur should this action again be tried. For the reasons previously set forth, that portion of the judgment of the trial court directing foreclosure and the sale of property described in the deed of trust is vacated. That portion of the judgment granting a directed verdict for the plaintiffs with regard to the defendants' counterclaim is reversed. That portion of the judgment granting a directed verdict in favor of the plaintiffs with regard to their original claim against the defendants is reversed. The cause is remanded for further proceedings in accordance with this opinion and applicable law.
Vacated in part, reversed in part and remanded.
ROBERT M. MARTIN and ERWIN, JJ., concur.
