Microsoft (R) Program Maintenance Utility Version 14.28.29335.0
Copyright (C) Microsoft Corporation.  All rights reserved.

The element 'package' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd' has invalid child element 'contentFiles' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd'. List of possible elements expected: 'files' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd'. This validation error occurred in a 'contentFiles' element.
NMAKE : fatal error U1077: '.\nuget.EXE' : return code '0x1'
Stop.