"value: "A node of type 'StartArray' was read from the JSON reader when trying to read a value of a property; however, a 'PrimitiveValue' or 'StartObject' node was expected."

Here is my ajax code to set multi choice field:

 $.ajax({
url: fullUrl,
method: "POST",
data: JSON.stringify({
  '__metadata': { 'type': 'SP.Data.AuditItem' },
  'Register': that.register,
  'RiskRegister': that.nextIndex,
  'Reopen': that.formatDate(that.reopen),
  'RiskOrIssue': that.riskOrIssue,
  'Locations': that.locations,               //<---multi-select choice field
  'ProblemT': that.probTitle,
  'ProblemStatement': that.problemStatement,
  'TaskOwner': that.taskOwner,
  'RiskOwner': that.riskOwner,
  'Auditor': that.auditor    //<--- multi-select choice field
}),
headers: {"accept": "application/json;odata=verbose","content-type": "application/json;odata=verbose","X-RequestDigest": $("#__REQUESTDIGEST").val()
},
success: function(){
  alert("Item Added!");
},
error: function(data){
  console.log(data);
}   
});    

For multi choice field, happen the error below, please help me how to solve:

"value: "A node of type 'StartArray' was read from the JSON reader when trying to read a value of a property; however, a 'PrimitiveValue' or 'StartObject' node was expected."

Thanks in advance