SharePoint Online Migration API Questions

I have an on-prem SP 2010 server and I'm trying to migrate data to SharePoint Online using the new API. When I run the ConvertTo-SPOMigrationTargetedPackage command the following error shows in the ConvertMigrationPackage log:

Warning Source XML file C:\package\LookupListMap.xml was not found

The last part of the log is this:

7/10/2015 4:20:27 PM Information Looking up Folder object '185dc0fb-f354-4abc-a6a8-001c85c1a59e' at ''
7/10/2015 4:20:27 PM Error Fatal Exception found: Value cannot be null.
Parameter name: uriString -