Cannot invoke "java.util.Map.put(Object, Object)" because "this.database" is null

При написании класса на Java возникла ошибка:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Map.put(Object, Object)" because "this.database" is null
        at database.DB.set(DB.java:14)
        at Main.main(Main.java:8)

Main.java

import database.*;

public class Main {
    public static void main(String[] args) {
        DB db = new DB();
    
        String[] table = db.table("name", "surname", "age");
        db.set("key", table, "Dima", "Krupin", 12);

        db.close();
    } 
}

database/DB.java

package database;

import java.util.Map;

public class DB {
    public Map<String, Map<String, Object>> database;

    public String[] table(String... fields) {
        return fields;
    }

    public void set(String key, String[] table, Object... column) {
        for (int i = 0; i < table.length; i++) {
            this.database.put(key, Map.of(table[i], column[i]));
        }
    }

    public Object get(String key) {
        return this.database.get(key);
    }

    public void close() {

    }
}

В чём ошибка?!