linked stack sum all - sum

Question

I am currently working through a problem where I have to implement a linked stack and write a method to compute the sum of its elements but my code will not work properly for the sum_all method. Any help is much appreciated
class Node:
def __init__(self, item = None, link = None):
"""creates a node with a given item and link"""
self.item = item
self.link = link
class LinkedStack:
def __init__(self):
self.top = None
self.stack_size = 0
def size(self):
return self.stack_size
def is_empty(self):
return self.stack_size == 0
def is_full(self):
# linked stacks are never full
return False
def push(self, item):
"""Places the given item at the top of the stack
if there is capacity, or raises an Exception."""
new_node = Node(item, self.top)
self.top = new_node
self.stack_size += 1
def pop(self):
"""Removes and returns the top element of the stack,
or raises an Exception if there is none."""
if self.is_empty():
raise Exception("The stack is empty")
item = self.top.item # get a reference to the top item before we remove the link to it
self.top = self.top.link
self.stack_size -= 1
return item
def sum_all(self):
current = self.top
sum = 0
while current is not None:
sum = sum + current.item
current = current.link
return sum
my_linked_stack = LinkedStack()
n1 = Node(1)
print(n1.item)
n2 = Node(2, n1)
print(n2.item, n2.link.item)
n3 = Node(3, n2)
print(n3.item, n3.link.item, n3.link.link.item)
my_linked_stack.push(n1)
my_linked_stack.push(n2)
my_linked_stack.push(n3)
print(my_linked_stack.sum_all()
returns
1
(2, 1)
(3, 2, 1)
Traceback (most recent call last):
File "/Applications/PyCharm CE.app/Contents/plugins/python-ce/helpers/pydev/pydevd.py", line 1496, in _exec
pydev_imports.execfile(file, globals, locals) # execute the script
File "/Users/evanchauhan/Desktop/technical interview lecture resources/2085 resources/Stacks and Queues implemented with arrays/linked_queues.py", line 106, in <module>
print(my_linked_stack.sum_all())
File "/Users/evanchauhan/Desktop/technical interview lecture resources/2085 resources/Stacks and Queues implemented with arrays/linked_queues.py", line 89, in sum_all
sum = sum + current.item
TypeError: unsupported operand type(s) for +: 'int' and 'instance'
I am trying to write a function named sum_all(self) that will compute the sum of all elements in my linked stack but I keep encountering this error which im not sure how to resolve