Cannot initialize disk in PowerShell : Initialize-Disk throws "The disk has already been initialized"

I'm trying to change the flash drive's partition style to GPT using PowerShell, but the cmdlet throws "The disk has already been initialized" (even though I've cleaned the drive):

PS C:\WINDOWS\system32> Clear-Disk 5 -RemoveData -RemoveOEMPS C:\WINDOWS\system32> Get-Disk 5Number Friendly Name                            OperationalStatus                          Total Size Partition Style          ------ -------------                            -----------------                          ---------- ---------------          5      UFD 2.0 Silicon-Power16G USB Device      Online                                       15.14 GB MBR     PS C:\WINDOWS\system32> Initialize-Disk 5 -PartitionStyle GPTInitialize-Disk : The disk has already been initialized.At line:1 char:1+ Initialize-Disk 5 -PartitionStyle GPT+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : NotSpecified: (StorageWMI:ROOT/Microsoft/Windows/Storage/MSFT_Disk) [Initialize-Disk], CimException+ FullyQualifiedErrorId : StorageWMI 41001,Initialize-Disk

How can I de-initialize the disk so that I can re-initialize it?

P.S. -PartitionStyle MBR shows the same error.