The generated C code contains errors

``` nimrod
type
  TOption*[T] = object
    case FIsSome: bool
    of false: nil
    of true: FData: T

proc some*[T](value: T): TOption[T] = TOption[T](FIsSome: true, FData: value)

when isMainModule:
  import unittest

  test "":
    var opt: TOption[seq[string]]
    opt = some(@[]) # this line causes the generated C code to contain errors
```
