Create AD:User with a UPN defined

I have a workflow using the createUserWithPassword action. This creates an AD:User object fine. Unfortunately the action doesn't create an AD User object with a UPN defined. Instead the user is just created with the UPN as a @...

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I duplicated the default action and modified it  to include the userPrincipalName in the parameters.

if(password){

  if(password != confirmPassword){

  throw "Unable to create a new user: Password not confirmed";

  }

}

try {

  container.createUserWithPassword(accountName, password, domainName , displayName, userPrincipalName);

}

catch (ex) {

  throw "Unable to create a new user: " + ex;

}

I added a scriptable task before the action runs to populate the userPrincipalName attribute and linked this to the action.

var userPrincipalName = accountName + "@corp.local";

System.log("User Principal Name for Account Creation is: " + userPrincipalName)

Unfortunately the accounts in AD still get created without a UPN. Any ideas how I create users with a UPN defined. The API explorer shows this as an option...

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Thanks