NSUserScriptTask unhappy about script format

I've got a trivial MacOS script:

#!/bin/sh

#  OpenInSafari.sh

open /tmp/GitNAVs.html

that I try to invoke with:

let bundle = Bundle.main
  if let script = bundle.url(forResource: "OpenInSafari", withExtension: "sh") {
  do {
  let scriptTask = try NSUserScriptTask(url: script)
  scriptTask.execute(completionHandler: nil)
  } catch {
  print(error)
  }

But I get:

Error Domain=NSCocoaErrorDomain Code=259 "The file “OpenInSafari.sh” couldn’t be opened because it isn’t in the correct format."

The *.sh file was created by Xcode and is where it's supposed to be in the bundle with read and write permissions. Any clue what's the problem?

Thanks,

Richard