Unknown crash in Swift 4.0 in iOS 9.3

I am facing very strange crash in my application. Application is working fine in > iOS 10 all version but it is crashing in iOS 9.x versions. I am not able to figure out why this is happening. It is not showing any kind of error in log console or in Zimbie or with evern try catch block.

I am dead lock over here. Any help would be highly appreciated. Here is the image with code with crash. I return multiple types like Int, Int32, Int64 and String but in all cases it is getting crash.

enter image description here

Unknown crash in Swift 4.0 in iOS 9.3