In X and Others v. Bulgaria the European Court of Human Rights acknowledged the violation by the Bulgarian State of Article 3 of the European Convention on Human Rights with regard to the procedural obligation to carry out effective investigations, as determined in the light of other treaties and in particular of the 2007 Lanzarote Convention on the Protection of Children against Sexual Exploitation and Sexual Abuse. The Court thus relied once again on the “vacuum” doctrine, according to which the provisions of the Convention must be interpreted in the light of other rules which are relevant ratione materiae (in the case under review, the Lanzarote Convention). The paper discusses whether by referring to the provisions of the Lanzarote Convention the Court impliedly resorted to systemic interpretation according to Article 31(3)(c) of the Vienna Convention on the Law of Treaties or whether the Court applied a different rule of interpretation.