Assigning BigInt stores wrong number (number+1)

Question

I want to define a BigInt number in JavaScript. But when I assign it, the wrong number is stored. In fact 1 is added to the number when storing.

let num = BigInt(0b0000111111111111111111111111111111111111111111111111111111111111)
console.log(num)                // Output: 1152921504606846976n
console.log(num.toString(2))    // Output: 1000000000000000000000000000000000000000000000000000000000000

So the number stored is 1152921504606846976, but it should be 11529215046068469765. Why is that?