Using tar on non-ascii file names yields different result from command line or Objective-C code

Question

I am using Objective C to archive some files using the tar command; some of them may or may not contain non-ascii characters, such as trademark sign ™, in Unicode U+2122.

I use NSTask to call the tar binary, as such:

NSArray * args = [NSArray arrayWithObjects: @"-czf",
                 [tar path], "@-C", [parentFolder path], fileName, nil];

NSTask * task = [NSTask launchedTaskWithLaunchPath: @"/usr/bin/tar" arguments:args];

The archive is created, but if a file inside it contains a non-ascii character, this file's name won't be preserved.

My problem is: if I run the following command line, the archive is created and all file names are preserved, even if it contains non-ascii characters:

tar -cvf archive.tar filename™

Now, where it gets really strange is if I put this exact command line in a sh script and run it from Objective-C using NSTask, it won't yield the same results as from the shell, i.e: the file names containing non-ascii characters won't be preserved upon being archived.

Using Zip resolves all the problems, but it would mean changing quite a bit of code, so I'm looking for a solution that would allow me to keep using tar first.

Thank you.