Assert::that()->numeric() being marked as "always evaluate to true"

For the following code:

```php
class Bar
{
    public function doBar(string $s)
    {
        Assert::that($s)->numeric();
    }
}
```

We're always getting the error:

```sh
# Call to method Assert\AssertionChain::numeric() will always evaluate to true.
```

I don't believe this is quite right, since we might have non-numeric strings, correct? Or are we doing something wrong?