New to Java

Hi, I have an example class SlowMap taken from a book, unfortunatelly it doesn't compile and throws this exception:

Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - Erroneous tree type: <any>
	at chapter17.SlowMap.entrySet(SlowMap.java:47)
	at java.util.AbstractMap.toString(AbstractMap.java:478)
	at java.lang.String.valueOf(String.java:2826)
	at java.io.PrintStream.println(PrintStream.java:771)
	at chapter17.SlowMap.main(SlowMap.java:54)
Java Result: 1

This is the code:

import java.util.*;

public class SlowMap<K, V> extends AbstractMap<K, V> {
    private List<K> keys = new ArrayList<K>();
    private List<V> values = new ArrayList<V>();
    
    @Override
    public V put(K key, V value) {
        V oldValue = get(key);
        if (!keys.contains(key)) {
            keys.add(key);
            values.add(value);
        } else
            values.set(keys.indexOf(key), value);
        return oldValue;
    }
    
    @Override
    public V get(Object key) {
        if (!keys.contains(key))
            return null;
        return values.get(keys.indexOf(key));
    }
    
    @Override
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> set = new HashSet<Map.Entry<K, V>>();
        Iterator<K> ki = keys.iterator();
        Iterator<V> vi = values.iterator();
        while (ki.hasNext())
            // this line throws exception
            set.add(new Map.Entry<K, V>(ki.next(), vi.next()));
        return set;
    }
    
    public static void main(String[] args) {
        SlowMap<String, String> m = new SlowMap<String, String>();
        m.putAll(Countries.capitals(6));
        System.out.println(m);
        System.out.println(m.get("BUŁGARIA"));
        System.out.println(m.entrySet());
    }
}

I don't understand the exception message, so I have no clue how to fix the code. Can anyone tell me what is wrong with this code?

Thanks,
PR.