Thread: Cannot push onto a stack

For some reason, whenever I try to push my objects onto a stack, I get a null pointer exception. This is what I have:

private Stack<BSTnode<K>> myStack;

private void preOrderTrav(BSTnode<K> node) {
if(node != null){
myStack.push(node); // I can't step past this step. This is where error occurs

while(!myStack.isEmpty()){
myStack.pop();
node = node.getLeft();
myStack.push(node.getRight());
myStack.push(node.getLeft());

}
}

...//more irreverent code down here

I get this error:
Exception in thread "main" java.lang.NullPointerException
at BSTSortedListIterator.preOrderTrav(BSTSortedListIt erator.java:14)

Does anyone know why I'm unable to push onto my stack?

Thanks