Are keySet entries of a WeakHashMap never null?

If I iterate over the key set of a WeakHashMap, do I need to check for null values?

WeakHashMap<MyObject, WeakReference<MyObject>> hm
    = new WeakHashMap<MyObject, WeakReference<MyObject>>();

for ( MyObject item : hm.keySet() ) {
    if ( item != null ) { // <- Is this test necessary?
        // Do something...
    } 
}

In other words, can elements of the WeakHashMap be collected while I am iterating over them?

EDIT

For the sake of this question, one can assume that no null entries is added in the hash map.