Cannot Insert Data into Excel File

I am trying to insert data into an excel file from a Nintex 2010 workflow, using a series of Call Web Service actions.

Test Scenario:

To eliminate any confusion I have created a blank excel file during this process.

First I call OpenWorkbookForEditing and store the SessionID in a variable to call whenever we want to set the cell value.

Next I call SetCell to set row 10, column 10 with a value of 100.

I then call CloseWorkbook to finish.

Whats happening

If I open the file, there is nothing in the cell I specified.  I know that the workflow is opening and closing the file, because I can see the SharePoint administrator in the Version History. The weird thing is that the file actually shrinks in size after the SetCell call is made. 

Also:

What format should the cellValue be for SetCell. It shows XML, but I don't see in the API or in your documentation how that should be formatted.  Any ideas?