Unable to index into an object of type System.ServiceProcess.ServiceController

Hello,

if I have executed the script on a server which run two instances SQL SERVER, no error message is returned

 $LIST_INST = (Get-Service -Name "MSSQL*" -Exclude "*Launcher*","*AD*")
foreach ($INST in $LIST_INST) {

      if ($CLUSSVC -eq $null -or $SCMODE -eq 'AUTO') {

        $INST_NAME = $INST.Name.REPLACE("MSSQL$","")
      }

      else {

        $INST_NAME = $INST.Name.REPLACE("SQL Server (","")
        $INST_NAME = $INST_NAME.REPLACE(")","")
        $CSERVICES = Get-WmiObject -Class win32_service -ComputerName $HOSTNAME | where { $_.Name -match "$INST_NAME" -and $_.Name -match 'MSSQLF' -or $_.Name -eq 'SQLBrowser' -or $_.Name -eq 'SQLWriter' }

      }
      $REGPATH = (Get-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL").$INST_NAME
      $key_name = "HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\" + $REGPATH + "\Cluster"
      $key_prop = Get-ItemProperty -Path $key_name -ErrorAction silentlycontinue
      $FULLPATH = "HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\$REGPATH"
      $SQLROOT = (Get-ItemProperty "$fullpath\Setup").SQLDataRoot

      if ($key_prop -eq $null) {
        $ser_name = (Get-WmiObject Win32_Computersystem).Name
      }
      else {
        $ser_name = $key_prop.ClusterName
      }

      $server = $SER_NAME + '\' + $INST_NAME
      if ($inst_name -eq "MSSQLSERVER") {
        $server = $ser_name
    
      }

if ($INST -ne $LIST_INST[-1]) {
                $line_FG = " { `"{#DBHOST}`":`"" + $HOSTNAME + "`" , `"{#INSTANCE}`":`"" + $INST_NAME + "`" , `"{#DBSPACE}`":`"" + $DBSPACE + "`"},"

              }

              else {
                $line_FG = " { `"{#DBHOST}`":`"" + "(" + $HOSTNAME + ")" + $SER_NAME + "`" ,`"{#INSTANCE}`":`"" + $INST_NAME + "`" , `"{#DBSPACE}`":`"" + $DBSPACE + "`"},"

              }
             
	      if ($INST -eq $LIST_INST[-1]) {
	      $line_FG = " { `"{#DBHOST}`":`"" + $HOSTNAME + "`" , `"{#INSTANCE}`":`"" + $INST_NAME + "`" , `"{#DBSPACE}`":`"" + $DBSPACE + "`"}"
	      if ($i -lt $nb_FG_DB-1 -or $j -lt $nb_FG-1) { $line_FG += ',' } 

              }
          
  
           Write-Host $line_FG

}

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But if I have executed the script on a server which run only one instance SQL SERVER, the following erreor message is returned

Unable to index into an object of type System.ServiceProcess.ServiceController.

+               if ($INST -ne $LIST_INST[ <<<< -1]) {
    + CategoryInfo          : InvalidOperation: (-1:Int32) [], ParentContainsErro
    + FullyQualifiedErrorId : CannotIndex
Thanks