In Practice quiz: Neural network implementation in Python in calculating

z2_1 = np.dot(w2_1, a1) + b2_1

it looks like the dot product is of vectors of different dimensions:

w2_1 = np.array( [-7, 8] ) is a 1D vector with 2 elements

while

a1 = np.array( [a1_1, a1_2, a1_3] ) - 1D vector with 3 elements.

Is it legal to dot-product these 2 vectors!?