Calling a method that accepts an optional error pointer

I'm trying to call a method in MacRuby that's defined like this:

NSPropertyListSerialization
---------------------------
+ (id)propertyListWithStream:(NSInputStream *)stream
    options:(NSPropertyListReadOptions)opt
    format:(NSPropertyListFormat *)format
    error:(NSError **)error

The last argument has to be a pointer, so I'm doing this:

err = Pointer.new '@'

data = NSPropertyListSerialization.propertyListWithStream plist,
  options: KCFPropertyListMutableContainers,
  format: KCFPropertyListBinaryFormat_v1_0,
  error: err
开发者_StackOverflow

However, I get this error:

expected instance of Pointer, got `200' (Fixnum) (TypeError)

The stack trace comes from the error: err line.

This error is the same regardless of the type of the object passed as error. It can be nil, a Pointer, a string or a completely arbitrary type, the error stays the same.

I have no idea where 200 fixnum comes from. Certainly not my code (there is barely any more code in this script than what I pasted).

Update: Solved with the help of @whitequark and @alloy. This was for editing Safari bookmarks in iOS Simulator. You can see how to read/write binary plist files here

Stacktraces for multiline statements always refer to the last line (i.e. if the error is not caused by a locateable subexpression), and fixnum 200 is one of the constants beginning with KCFPropertyList.

Depending on the contents of the plist, you should be able to read it with: NSDictionary.dictionaryWithContentsOfFile(path).