Consider the following Python code implementing Simpson's 3/8 rule for numerical integration:

def simpsons_38_rule(f, a, b, n):

    h = (b - a) / n

    integral = f(a) + f(b)

    for i in range(1, n):

        x = a + i * h

        if i % 3 == 0:

            integral += 4 * f(x)

        else:

            integral += 3 * f(x)

    integral *= 3 * h / 8

    return integral

What is the primary issue with this implementation?