def dot_product(v, u):
    """ (list of num, list of num) -> num
        Returns the dot product of the two vectors v and u.
        Precondition: len(v) == len(u)
    """
    try:
        if v == [] and u == []:
            return 0
        else:
            return v[0]*u[0] + dot_product(v[1:], u[1:])
    except IndexError:
         # the length of the vectors not the same
         print('The length of the vectors is not the same')
         return None
    except TypeError:
         # There is an incorrect type used
         print("Incorrect type!")
         return None