Measurements of ac magnetic susceptibility and dc resistivity of a high-quality single-crystal MnSi were carried out at high pressure making use of helium as a pressure medium. The form of the ac magnetic susceptibility curves at the magnetic phase transition suddenly changes upon helium solidification. This implies strong sensitivity of magnetic properties of MnSi to nonhydrostatic stresses and suggests that the early claims on the existence of a tricritical point at the phase-transition line are probably a result of misinterpretation of the experimental data. At the same time resistivity behavior at the phase transition does not show such a significant influence of helium solidification. The sharp peak at the temperature derivative of resistivity, signifying the first-order nature of the phase transition in MnSi successfully survived helium crystallization and continued the same way to the highest pressure.