Delegation and generics craziness

So,

I am trying to create a delegate Map that simply throws an exception
if an attempt to insert a duplicate key occurs. That code is simple.
However, the delegate code is not and I am getting compile errors with
regards to my generics. I make no claims to fully understand generics.
So, I present the work I have with the hopes someone can explain to me
why this won't work.

Christian
http://christan.bongiorno.org

public class ExceptionOnDuplicateKeyMap<K, V> implements Map<K,V> {
    private final Map<? extends K, ? extends V> delegate;

    public ExceptionOnDuplicateKeyMap(Map<? extends K, ? extends V>
delegate) {
        this.delegate = delegate;
    }

    public void clear() {
        delegate.clear();
    }

    public boolean containsKey(Object key) {
        return delegate.containsKey(key);
    }

    public boolean containsValue(Object value) {
        return delegate.containsValue(value);
    }

    public Set<Entry<K, V>> entrySet() {
        return delegate.entrySet(); // error here
    }

    public V get(Object key) {
        return delegate.get(key);
    }

    public boolean isEmpty() {
        return delegate.isEmpty();
    }

    public Set<K> keySet() {
        return delegate.keySet(); // error here
    }

    public V put(K key, V value) {
        if(delegate.containsKey(key))
            throw new IllegalArgumentException();
        return delegate.put(key,value); // error here
    }

    public void putAll(Map<? extends K, ? extends V> m) {
        for (Entry<? extends K, ? extends V> entry : m.entrySet())
            put(entry.getKey(),entry.getValue());
    }

    public V remove(Object key) {
        return delegate.remove(key);
    }

    public int size() {
        return delegate.size();
    }

    public Collection<V> values() {
        return delegate.values(); // error here
    }

}