Working with array and Dictionaries after loading them from Disk using np.load and np.save

I have several huge arrays, and I am using np.save and np.load to save each array or dictionary in a single file and then I reload them, in order not to compute them another time as follows.

save(join(dir, "ListTitles.npy"), self.ListTitles)
self.ListTitles = load(join(dir,"ListTitles.npy"))

The problem is that when I try to use them afterwards, I have errors like (field name not found) or (len() of unsized object). For example:

 len(self.ListTitles) or when accessing a field of a dictionary return an error.

I don't know how to resolve this. Because when I simply use this code, it works perfectly:

M = array([[1,2,0], [3,4,0], [3,0,1]])
vector = zeros(3529)
save("M.npy", M)
save("vector.npy", vector)
vector = load("vector.npy")
B = load("M.npy")
print len(B)
print len(vector)