Why these two nearly the same bit-shift functions produce different answers?

Question

Using unsigned to represent a mask for first len bits, I wrote a function as below:

void left_shift(int len) {
  unsigned x = ~0u << (32 - len);
  printf("%08x\n", x);
}

However, when I call left_shift(0) the console prints ffffffff, rather than 00000000 that I expected.

So, I made another function to test whether I was misunderstanding the bit-shift operation as below:

void left_shift() {
  unsigned x = ~0u << (32 - 0);
  printf("%08x\n", x);
}

This time, it prints the expected 00000000.

To conclude:

int main() {
  left_shift();   // prints "00000000"
  left_shift(0);  // prints "ffffffff"
  return 0;
}

I'm really confusing. Could anyone help explain?