Šulla was called up for two unofficial friendly fixtures in January 2017. He made his debut on 8 January 2017 against Uganda, conceding two goals in the first half - by Moses Oloya and Farouk Miya. Slovakia went on to lose 1–3. Šulla did not play in the 0–6 loss against Sweden later that week. He earned another cap in the final match of the 2018 King's Cup, on 25 March 2018, in a 3–2 victory over Thailand, despite being benched in the 2–1 semi-final victory over UAEthree days earlier. In the 42nd minute Šulla conceded a controversial goal; an attempted pass to Róbert Mazáň went straight to the foot of Teerasil Dangda, who found Jakkaphan Kaewprom, who enjoyed a de factoempty net to score to - 2–1.
Ján Kozák's resignation incident
Šulla was nominated for a double fixture against Czech Republic and Sweden on 13 October and 16 October 2018, respectively. He did not play in either of the two matches, but was involved in the controversy, that had led to resignation of Ján Kozák, then Slovakia's longest serving and most successful coach, who introduced Šulla to the national team. Šulla violated the code of conduct of national team player, along with Martin Dúbravka, Norbert Gyömbér, Ľubomír Šatka, Milan Škriniar, Vladimír Weiss and Stanislav Lobotka. On the night of 13 October 2018, after loss in Slovakia's second UEFA Nations League fixture and a derby match against Czech Republic, they left the hotel and went out, missed the bedtime and violated the wellness policy, regarding regeneration and rehabilitation. They were allegedly busted by Ján Kozák at about midnight, admitting to the incident and apologising during the next day. Ján Kozák resigned from the national team in afternoon hours of 14 October, revealing the details of his decision in a press conference on 18 October, to avoid distractions during the preparation for the fixture against Sweden, that was managed by his former assistant Štefan Tarkovič on a caretaker basis. Kozák cited his inability to work with the squad under such conditions as the primary reason. President of SFZ, Ján Kováčik, said that the players will be punished by not receiving any financial rewards for any future nominations and performances in the national team during the upcoming qualification cycle. Šulla was additionally criticised by the vice-president of Slovan Bratislava, Ivan Kmotrík jr., stating financial sanctions and a possible temporary relegation to the reserve squad as options of punishment, for poor representation of the club and violation of the wellness policy.
