I'm trying to move a custom field from one sandbox into another using the Force.com Migration Tool, and getting a funky "Cannot rename standard profile" error.  

Here's my package file which goes in the "$PROJECT_ROOT/unpackaged" folder:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>*</members>
        <name>Profile</name>
    </types>
    <types>
      <members>Account.Test__c</members>
      <name>CustomField</name>
    </types>
    <version>18.0</version>
</Package>

Here's my ant build.xml file which sits in the $PROJECT_ROOT folder:

<project name="Sample usage of Salesforce Ant tasks" default="test" basedir="." xmlns:sf="antlib:com.salesforce">
    <property file="build.properties"/>
    <property environment="env"/>

    <target name="retrieveUnpackaged">
      <mkdir dir="retrieveUnpackaged"/>
      <sf:retrieve username="${sf.username}" password="${sf.password}" serverurl="${sf.serverurl}" retrieveTarget="retrieveUnpackaged" unpackaged="unpackaged/package.xml"/>
    </target>

    <target name="deployUnpackaged">
      <sf:deploy username="${sf.deploy.username}" password="${sf.deploy.password}" serverurl="${sf.deploy.serverurl}" deployRoot="retrieveUnpackaged"/>
    </target>
</project>

This properties file all goes in the $PROJECT_ROOT folder:

# build.properties
#

sf.username = <username>@<org>.sandbox1
sf.password = <password>
sf.serverurl = https://test.salesforce.com

sf.deploy.username = <username>@<org>.sandbox2
sf.deploy.password = <password>
sf.deploy.serverurl= https://test.salesforce.com

To run, execute the following two commands:

ant retrieveUnpackaged
ant deployUnpackged

When I try to deploy to the second sandbox I am getting the following error:

Error: profiles/Guest License User.profile(Guest License User):Cannot rename standard profile

My package is pretty straightforward and I can't imagine what I might be doing to get this error.  Any thoughts?

Thanks,

Greg