Many rubble mound breakwaters are nowadays made with an armour layer consisting of single layer interlocking elements. The stability of these armour layers could well be influenced by the irregularity of the rock underlayer, as that influences the degree of interlocking. Especially for the new types of regularly placed armour, this might be an important factor for the stability. However, this aspect has not been studied widely yet. Therefore, this paper investigates the influence of irregularities in the underlayer on the stability of a type of single layer breakwater elements. Model tests have been conducted in which the irregularities in the underlayer were systematically varied. The irregularities in the underlayer and the orientations of breakwater elements were measured with 3D-scanning. The breakwater armour unit used in the tests is the Xbloc^Plus. A new failure mechanism, not previously observed for breakwater elements, was found to initiate damage. Causing the armour to be pushed outward by a combined effect of the weight of the upper armour and the excessive hydraulic pressure of the remaining water under the armour layer. Especially large-scale convex (i.e. protruding outwards) undulations in the cross-shore direction influenced the stability of the armour layer. This influence has been quantified.