Null Pointer in Red Black Tree implementation

Question

I have a red black tree implementation that I have implemented from pseudo code found in the CLRS and I'm running into a NPE in the insert fixup procedure. Here is my implementation in java. (Ignore the variable names, I know it's not convention)

private RbtNode root;
    private RbtNode nil;
    private static final boolean BLACK = true;
    private static final boolean RED = false;

    public RedBlackTreeImpl() {
        nil = new RbtNode("");
        nil.color = BLACK;
        root = nil;
    }

    @Override
    public void rbInsert(T data) {
        RbtNode z = new RbtNode(data);
        RbtNode y = this.nil;
        RbtNode x = this.root;
        while(x != this.nil){
            y = x;
            if(z.data.toString().compareTo(x.data.toString()) < 0){
                x = x.left;
            } else {
                x = x.right;
            }
        }
        z.parent = y;
        if(y == this.nil) {
            this.root = z;
        } else {
            y.right = z;
        }
        z.left = this.nil;
        z.right = this.nil;
        z.color = RED;
        rbInsertFixup(z);
    }

    @Override
    public void rbInsertFixup(RbtNode z) {
        while(z.parent.color == RED){
            if(z.parent == z.parent.parent.left){
                RbtNode y = z.parent.parent.right;
                if(y.color == RED){
                    z.parent.color = BLACK;
                    y.color = BLACK;
                    z.parent.parent.color = RED;
                    z = z.parent.parent;
                } else if(z == z.parent.right){
                    z = z.parent;
                    leftRotate(z.parent.parent);
                }
                z.parent.color = BLACK;
                z.parent.parent.color = RED;
                rightRotate(z.parent.parent);
            } else {
                RbtNode y = z.parent.parent.left;
                if(y.color == RED){
                    z.parent.color = BLACK;
                    y.color = BLACK;
                    z.parent.parent.color = RED;
                    z = z.parent.parent;
                } else if(z == z.parent.left){
                    z = z.parent;
                    rightRotate(z.parent.parent);
                }
                z.parent.color = BLACK;
                z.parent.parent.color = RED;
                leftRotate(z.parent.parent);
            }
        }
        this.root.color = BLACK;
    }

    @Override
    public void leftRotate(RbtNode x) {
        RbtNode y = x.right;
        x.right = y.left;
        if(y.left != this.nil) {
            y.left.parent = x;
        }
        y.parent = x.parent;
        if(x.parent == this.nil){
            this.root = y;
        } else if(x == x.parent.left){
            x.parent.left = y;
        } else {
            x.parent.right = y;
        }
        y.left = x;
        x.parent = y;
    }

    @Override
    public void rightRotate(RbtNode x) {
        RbtNode y = x.left;
        x.left = y.right;
        if(y.right != this.nil) {
            y.right.parent = x;
        }
        y.parent = x.parent;
        if(x.parent == this.nil){
            this.root = y;
        } else if(x == x.parent.right){
            x.parent.right = y;
        } else {
            x.parent.left = y;
        }
        y.right = x;
        x.parent = y;
    }

Node Class

public class RbtNode<T> {

    T data;
    boolean color;                  //true indicates black; false indicates red
    RbtNode<T> left;
    RbtNode<T> right, parent;

    public RbtNode(T data) {
        if (data == null) {
            throw new IllegalArgumentException("Data cannot be null");
        }
        this.data = data;
    }

    public boolean getColor() {
        return this.color;
    }

    public T getData() {
        return this.data;
    }

    public RbtNode<T> left() {
        return left;
    }

    public RbtNode<T> right() {
        return right;
    }

    public RbtNode<T> parent() {
        return parent;
    }

}

If run a test insert on it like such:

       RedBlackTreeImpl rbt = new RedBlackTreeImpl();
       rbt.rbInsert("a");
       rbt.rbInsert("k");
       rbt.rbInsert("q");
       rbt.rbInsert("c");
       rbt.rbInsert("j");
       rbt.rbInsert("e");

The NPE occurs when attempting to insert "c". Specifically at in rbInsertFixup on the line z.parent.parent.color = RED; immediately before the rightRotate within the first if of the while loop. In that instance z is root, so root's parent is nil, and so it's trying to get nil's parent which is actually null.

I tried making nil's left, right, and parent nodes equal to nil, but that caused an infinite loop. I think this comes down to a lack of concepts issue. So I suppose my question could be broken into an either or question. Is the fact that I'm getting a NPE a sign that I should continue the process, or have I set up nil incorrectly? What is it that I'm missing?