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Issue Description:

When running the following code on a device or a simulator (in release build configuration) the app crashes with "EXC_BAD_ACCESS".

The problem was found by accident, and even though the code should not be written like this, someone else might run into the same (or similar) issue.

Below is a trimmed down version of the original code, which exhibit the problem:

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        save(value: 0)
    }

    func save(value: Double?) {
        var params: [[String : Any]] = [["a": 0]]
        params = [[
            "b": 0,
            "c": value.map(String.init) as Any
            ]]
    }
}

I have tried doing the following modifications to the code, which all make the problem go away:

1. Changing the type of save()'s `value` parameter

2. Removing the key "b"

3. Removing the key "c"

4. Removing `.map(String.init)` from the key "c"

5. Moving `value` to a local variable

6. Setting the initial value of `params` to an empty list `[]`

7. Changing to `let params` and assign the second value directly

Modifications that does not make a difference:

1. The type of the value in key "a"

2. Names of the keys

3. Removing the cast to `Any` (Will prompt a warning)

I was not able to reproduce the problem in the Xcode playground, but I guess that is because the playground runs in debug mode.