Im trying to find active futures for my site from below script but im prompt with error ( cannot call a method on a null-valued expression) 
Appreciate your help  


    $activeFeatures = Get-SPFeature -Site --site _URL--
    $allSiteFeatures | ForEach-Object 
    {
    $active = $false
        foreach($feature in $activeFeatures)
        {
          if($feature.Id.ToString() -eq $_.Id.ToString())
          {
             $active = $true
          }
        }
        if($active)
        {
           $_.DisplayName
        }
    }


> ERROR----------- You cannot call a method on a null-valued expression.
> //At line:5 char:51 //+       if($feature.Id.ToString() -eq
> $_.Id.ToString <<<< ())    // + CategoryInfo          :
> InvalidOperation: (ToString:String) [], RuntimeE    xception    +
> FullyQualifiedErrorId : InvokeMethodOnNull