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Hint:
When the weights are zeros, then the sigmoid of z will always be sigmoid(0) = 0.5. It’s the same value for both the true and false cases. When you do the math to compute the cost, that will give a value of 0.693 (because that’s the natural log of 0.5).

Note also the following mathematical relationship:

0.693 * 2 = 1.386

Are you sure that your code faithfully implements the mathematical formula for the cost? The above relationship seems like a clue about the nature of the problem.