Testing BizTalk Maps that use Flat File Schemas

I was testing a BizTalk map that used a flat file schema with a flat file instance input file and ran into this error:

Error 3 Output validation error: Data at the root level is invalid. Line 1, position 1.

Turns out that the map is treating the input file as an XML file. In the map’s property, ensure the TestMap Input is set to Native.