In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang et al. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.