OpenXMLValidator requires an opened file, but the file won't open - openxml-sdk

Question

I've been struggling with how to fix a SpreadsheetML file being written by a 3rd party library. After some poking about I came across this blog post on OpenXMLValidator. The code is very simple:
Using testdoc As SpreadsheetDocument = SpreadsheetDocument.Open(filepath, False)
Try
Dim validator As New OpenXmlValidator()
Dim count As Integer = 0
For Each [error] As ValidationErrorInfo In validator.Validate(testdoc)
count += 1
Console.WriteLine("Error " & count)
Console.WriteLine("Description: " & [error].Description)
Console.WriteLine("ErrorType: " & [error].ErrorType)
Console.WriteLine("Node: " & [error].Node.ToString())
Console.WriteLine("Path: " & [error].Path.XPath)
Console.WriteLine("Part: " & [error].Part.Uri.ToString())
Console.WriteLine("-------------------------------------------")
Next
Catch ex As Exception
Console.WriteLine(ex.Message)
End Try
testdoc.Close()
End Using
One teenie problem, however. When SpreadsheetDocument.Open runs, it throws an error, System.InvalidOperationException: 'Specified part does not exist in the package.' and thus the Validator can't examine it. This is... odd, given that not being able to open the package is precisely the problem I'm trying to solve!
Does anyone have any hints or tricks here? Or perhaps an alternate solution to tracking down OpenXML errors?