"A process in the process pool was terminated abruptly" error on concurrent.futures.ProcessPoolExecutor()

Question

I'm trying to multiply matrices using multiprocessing to see how much faster it can run than a regular synchronous process. Here is the function:

def multvecs(inp):
        vec1, vec2, index = inp
        #i, j = index
        sum = 0
        for k in range(vec1.shape[0]):
            sum += vec1[k] * vec2[k]
        #prod[i][j] = sum
        return sum, index

def asyncmult(m1, m2):
    threads = []
    prod = np.zeros((m1.shape[0], m2.shape[1]))

    with concurrent.futures.ProcessPoolExecutor() as executor:
        for i in range(m1.shape[0]):
            for j in range(m2.shape[1]):
                f = executor.submit(multvecs, (m1[i], m2[:, j], (i, j)))
                threads.append(f)
        for f in concurrent.futures.as_completed(threads):
            vprod, index = f.result()
            i, j = index
            prod[i][j] = vprod
    return prod
     

I'm running this on windows so I protected the entire main with if name == 'main'.

concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending

This is the error message. how do I fix this? I tried saving the file as well as moving multvecs() to outermost scope.