Why does my NumPy function not work out the cosine of two vectors?

Question

I have this function to work out the cosine between two vectors, except it isn't working for some reason.

def length(x):
    return math.sqrt(x.dot(x))


def unitVectorNormalize(u):
    if length(u) == 0:
        return "Your vector is null."
    return u / length(u)


def cosine(vector1, vector2):
    return vector1.dot(vector2) / (unitVectorNormalize(vector1) * unitVectorNormalize(vector2))

Is there something I am doing wrong?

I am aware that there is a NumPy method to figure out the cosine similarity, but I am told (for my course) to write my own fucntion.