Garbagecollect funciton error with excel

Hi,

I am preparing a script to work with excel sheet. The thing is when I try to use Garbagecollect I get an error.


This is the error message:

Exception calling "ReleaseComObject" with "1" argument(s): "Object reference not set to an instance of an object."
At E:\Preparacion\scripts\CASINO-CGM-pruebas.ps1:522 char:64
+     ([System.Runtime.Interopservices.Marshal]::ReleaseComObject <<<< ([System.__ComObject]$ref) -gt 0)
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException


An extact of the script code:
you can see the code below. Line 522 is:
... ReleaseComObject([System.__ComObject]$ref) -gt 0)..


---------------------------------------------------------------------

Function GarbageCollect ([object]$ref){
    ([System.Runtime.Interopservices.Marshal]::ReleaseComObject([System.__ComObject]$ref) -gt 0)
    [System.GC]::Collect()
    [System.GC]::WaitForPendingFinalizers()
}

 $a = GarbageCollect($c)
 $a = GarbageCollect($Wb)
 $a = GarbageCollect($xl)

--------------------------------------------------------------------


Please, I would thank your help. I am absolutelly lost with this issue. I have used that function before, however, I dont sure what is happening in this case