Merge And Sort Function with issues in certain pester versions?

MergeAndSort.ps1

Function MergeAndSort {
    <#
        .SYNOPSIS
            MergeAndSort

        .DESCRIPTION
            Merges arrays and sorts, extracted for refactoring and adding unit tests

        .PARAMETER VMNumbers
            VMware Number array

        .PARAMETER ADNumbers
            ActiveDirectory Number array

        .PARAMETER DNSNumbers
            DNS Number array

        .PARAMETER SCCM2012Numbers
            ConfigMgr Number array

        .PARAMETER SCOM2007Numbers
            SCOM Number array

        .EXAMPLE
            PS C:\> MergeAndSort `
                -VMNumbers $VMNumbers `
                -ADNumbers $ADnumbers `
                -DNSNumbers $DNSNumbers `
                -SCCM2012Numbers $SCCM2012Numbers`
                -SCOM2007Numbers $SCOM2007Numbers

        .NOTES
            For additonal information please contact ffff@ffff.com

        .LINK
            https://fffff
    #>
    [CmdletBinding()]
    Param(
        [string[]]$VMNumbers,

        [string[]]$ADNumbers,

        [string[]]$DNSNumbers,

        [string[]]$SCCM2012Numbers,

        [string[]]$SCOM2007Numbers
    )

    # Merge arrays
    $mergedList = New-Object System.Collections.ArrayList

    [void]$mergedList.Add(0)

    if ($VMNumbers) { [void]$mergedList.AddRange($VMNumbers) }
    if ($ADnumbers) { [void]$mergedList.AddRange($ADnumbers) }
    if ($DNSNumbers) { [void]$mergedList.AddRange($DNSNumbers) }
    if ($SCCM2012Numbers) { [void]$mergedList.AddRange($SCCM2012Numbers) }
    if ($SCOM2007Numbers) { [void]$mergedList.AddRange($SCOM2007Numbers) }

    [System.Collections.ArrayList]$mergedList = $mergedList | Sort-Object -Unique

    Return $mergedList
}

# This test would work on windows 10.. (pester 4.0.3) but not on windows 7 same pester version, wmf 5.1
It "sorts numbers and removes duplicates" {
    MergeAndSort `
        -VMNumbers @(200, 100, 300) `
        -ADNumbers @(200, 100, 300) `
        -DNSNumbers @(200, 100, 300) `
        -SCCM2012Numbers @(200, 100, 300) `
        -SCOM2007Numbers @(200, 100, 300) | Should Be @(0,100,200,300)
}

# modifying to this doesn't help
It "sorts numbers and removes duplicates" {
    $results = MergeAndSort `
        -VMNumbers @(200, 100, 300) `
        -ADNumbers @(200, 100, 300) `
        -DNSNumbers @(200, 100, 300) `
        -SCCM2012Numbers @(200, 100, 300) `
        -SCOM2007Numbers @(200, 100, 300)
    ,$results | Should Be @(0,100,200,300)
}