Python object attribute is another object, won't load using pickle.load(). What am I doing wrong?

I have an object:

class X():
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

whose attribute c, is a of list objects (of a different kind):

class Y():
    def __init__(self, x, y):
        self.x = x
        self.y = y

I pickle this as follows:

import pickle
pickle.dump(instance_of_class_X,open(dir, "wb"))

I load as follows:

import pickle
from some_library import X, Y # I import the two classes involved
pickle.load(open(dir,"rb"))

I get the following error:

AttributeError: 'module' object has no attribute 'Y'

Not sure what to do, any help would be highly appreciated.