NATO Secretary General Jens Stoltenberg called on Macedonians on Thursday to vote in a referendum to embrace a deal on its name with neighboring Greece and join the defense alliance, Reuters reported.

Macedonia has set Sept. 30 for the referendum on NATO and European Union membership bids and on its agreement with Greece on the country’s name.

In June, NATO invited Macedonia to begin accession talks with the alliance, but to finalize membership talks it will have to adopt constitutional changes.

Nationalists, including President Gjorge Ivanov, oppose the deal saying it is against the constitution.