Error when enumerating through site columns

Question

I'm working on a Sharepoint 2010 site. I am trying to run this script from a website in order to create an xml containing info on my site columns.

$sourceWeb = Get-SPWeb http://farmstuff:1284
$xmlFilePath = "C:\Users\User\Scripts\Script-SiteColumns.xml"

#Create Export Files
New-Item $xmlFilePath -type file -force

#Export Site Columns to XML file
Add-Content $xmlFilePath "<?xml version=`"1.0`" encoding=`"utf-8`"?>"
Add-Content $xmlFilePath "`n<Fields>"
$sourceWeb.Fields | ForEach-Object {
   if ($_.Group -eq "Custom Columns") {
        Add-Content $xmlFilePath $_.SchemaXml
    }
}
Add-Content $xmlFilePath "</Fields>"

$sourceWeb.Dispose()

And I get the following error:

An error occurred while enumerating through a collection: Exception has been thrown by the target of an invocation..
At C:\Users\User\Scripts\get-sitecolumns.ps1:10 char:5
+     <<<< %sourceWeb.Fields | ForEach-Object {
    + CategoryInfo           : InvalidOperation: (Microsoft.Share...on+SPEnumerator:SPEnumerator) [], Runtime Exception
    + FullyQualifiedErrorId: BadEnumeration

Any help would be appreciated! I am not too familiar with Sharepoint or Powershell nuances yet. Thanks.

Edit:

<Field ID="{3C0E9E00-8FCC-479f-9D8D-3447CDA34C5B}" Name="OtherAddressCountry" StaticName="OtherAddressCountry" SourceID="http://schemas.microsoft.com/sharepoint/v3" DisplayName="Other Address Country" Group="Core Contact and Calendar Columns" Type="Text" Sealed="TRUE" AllowDeletion="TRUE" />
<Field ID="{9EBCD900-9D05-46c8-8F4D-E46E87328844}" Name="Categories" StaticName="Categories" SourceID="http://schemas.microsoft.com/sharepoint/v3" DisplayName="Categories" Group="Base Columns" Type="Text" Sealed="TRUE" AllowDeletion="TRUE" />

Answer 1

Are you flexible and try the following method

foreach ($field in $web.Fields)
{
  if($field.Group -eq "Custom Column Group")
  {
     DO YOUR THING
  }
}