SortOrder for Contact model display label not giving value

[vaibhav.sood]

sortOrders:[
                SortOrder {
                                detail: ContactDetail.Name
                                caseSensitivity: Qt.CaseInsensitive
                                field:Name.DisplayLabel
                                direction: Qt.AscendingOrder
                            }
]

sortOrders:[
                SortOrder {
                                detail: ContactDetail.Name
                                caseSensitivity: Qt.CaseInsensitive
                                field:Name.DisplayLabel
                                direction: Qt.AscendingOrder
                            }
]

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This is a sort order being used for sorting the contact model based on the display label. however the contacts are not getting sorted.

Name.FirstName

Name.FirstName

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and

Name.LastName

Name.LastName

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work perfectly but display label is not being read . Any help is much appreciated.