Powershell script issue

Hi There

I am running the below script and getting error running on any server 2012 r2 and up but works on workstations.

Method invocation failed because [System.Object[]] does not contain a method named 'GetVersion'.

Function Get-OSVersion{
     $signature = $Signature = @"

 [DllImport("kernel32.dll")]

 public static extern uint GetVersion();

"@
    Add-Type -MemberDefinition $signature -Name "Win32OSVersion" -Namespace Win32Functions -PassThru
    }

$os = [System.BitConverter]::GetBytes((Get-OSVersion)::GetVersion())

Write-Output $os