108 Swider and den Heijer interpretative difficulty is the question of what types of political entities should be considered as a ‘State’ for the purposes of satisfying the definition, and whether it includes failed States, for example, or States that may not have been recognised by a State party.22 Especially liable to divergent interpretation, further, is the condition that a person is ‘not considered as a national […] under the operation of [...] law’. It is obvious that substantiating statelessness can involve serious evidentiary challenges, precisely because statelessness is often not documented. Normally, it will require probing into how a State applies its nationality laws and possible decisions taken in respect of the individual. But how far this investigative duty stretches, and whether, for example, States are also required to engage in dialogue with the other States in order to determine how the laws are interpreted and applied, is not specified in the Convention. Disparities in state practice exist in spheres such as the standard of proof and the burden of proof in producing documentary evidence.23 State Parties also have diverging practices on the question whether individuals who voluntarily renounce their nationality should be considered stateless.24 It is important to note that in the definition of a stateless person of the 1954 Convention, such factors as the migration status of the individual or his ability to access basic rights do not play a role. Some authors, by invoking the socalled concept of ‘de-facto’ statelessness, broaden this definition to include persons without legal residence, who cannot be deported or who lack access to some basic rights, even if another State considers them as nationals.25 We believe that even though statelessness is closely linked to the legal vulnerability of undocumented migrants and their discrimination and exploitation, these concepts cannot be conflated, and require separate legal solutions. Not all undocumented migrants are stateless, and not all stateless persons are undocumented migrants. We therefore focus on statelessness in the legal 22 This holds for people who are considered as nationals by entities that do not enjoy universal recognition as states, such as Taiwan or Palestine. 23  Batchelor, C., ‘The 1954 Convention Relating to the Status of Stateless Persons: Implementation Within the European Union Member States and Recommendations for Harmonization’, 22(2) Refuge (2005) 40. Also see UNHCR, Handbook on Protection of Stateless Persons (Geneva, UNHCR, 2014), para 87–93. 24 More extensively: UNHCR, Handbook on Protection of Stateless Persons (Geneva, UNHCR, 2014), para 51; Chassin, C.-A., ‘Panorama français du droit de l’apatridie’, 2 Revue française de droit administratif (2003) 324–330. 25 See, for example Sawyer, C. & B.K. Blitz (eds), Statelessness in the European Union: Displaced, Undocumented, Unwanted (Cambridge, Cambridge University Press, 2011); Busser, A. & P.R. Rodrigues, ‘Staatloze Roma in Nederland’, 8 Asiel-en Migrantenrecht (2010) 384–390. European Journal of Migration and Law 19 (2017) 101–135