Confusion about `TypeMustMatch` and f64

[legendre6891]

Hi! I have a question about why the following code seems to work without throwing an assert error.

On the Rust side:

#[pyfunction]
fn sum_up<'py>(py: Python<'py>, array: PyArrayLike1<'py, f64, TypeMustMatch>) -> f64 {
    array.as_array().sum()
}

On the python side

>>> A = np.array([2**63, 2**63+1], dtype=np.uint64)
>>> sum_up(A)
1.8446744073709552e+19

It seems like A is being converted to a f64, and then addition is carried out. (For reference, A.sum() = 1.) I thought that the TypeMustMatch means that the array's dtype must be f64.

Either I'm misunderstanding TypeMustMatch or perhaps there is a bug here?