Sequence with linked list and Generics *2 Errors*

Hello! Ran the J-unit tests, runs 19/19 with 2 Errors located in my methods: addAll & concatenation.
I truly appreciate the help friends!

Code java:

public void addAll(LinkedSeq<E> addend)
	   {
		   if(addend == null)
		   {
			   throw new NullPointerException("addend is null");
		   }
 
		   ********tail.setLink(addend.head);********(Seems it originates here)
 
		   //addend.head = this.tail;
		   tail = addend.tail;
		   //addend.tail = this.tail;
		   addend.cursor = null;
		   manyNodes += addend.manyNodes;
 
		   addend.tail = tail; 
		   //if(addend.tail.getLink() != null)
		   //{
			  // addend.tail = addend.tail.getLink();
		   //}
	   }

And here's the second method

Code java:

public static <E> LinkedSeq<E> concatenation(LinkedSeq<E> s1, LinkedSeq<E> s2)
	   {
		   if((s1 == null) || (s2 == null))
		   {
			   throw new IllegalArgumentException ("concatenation:  one argument is null");
		   }
 
		   LinkedSeq<E> answer = new LinkedSeq<E>();
	       ****************answer.addAll(s1);*****************(Other trace)
	       answer.addAll(s2);
	       return answer;
	   }