title: Unable to pack a Nuget Package on VSTS Unable to cast object of type 'System.String' to type 'NuGet.Frameworks.NuGetFramework'
body: <p>On running 'pack' I am getting the following error...</p>
  
  <pre><code>2017-07-05T10:37:09.7762333Z ##[section]Starting: NuGet pack
  2017-07-05T10:37:09.7762333Z ==============================================================================
  2017-07-05T10:37:09.7762333Z Task         : NuGet
  2017-07-05T10:37:09.7762333Z Description  : Restore, pack, or push NuGet packages, or run a NuGet command. Supports NuGet.org and authenticated feeds like Package Management and MyGet.
  2017-07-05T10:37:09.7762333Z Version      : 2.0.3
  2017-07-05T10:37:09.7762333Z Author       : Microsoft Corporation
  2017-07-05T10:37:09.7762333Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkID=613747)
  2017-07-05T10:37:09.7762333Z ==============================================================================
  2017-07-05T10:37:10.9664575Z Downloading: https://dist.nuget.org/win-x86-commandline/v4.0.0/nuget.exe
  2017-07-05T10:37:11.0954621Z Caching tool: NuGet 4.0.0 x64
  2017-07-05T10:37:11.1084622Z Using version: 4.0.0
  2017-07-05T10:37:11.1094622Z Found tool in cache: NuGet 4.0.0 x64
  2017-07-05T10:37:11.1214629Z [command]C:\Windows\system32\chcp.com 65001
  2017-07-05T10:37:11.1634626Z Active code page: 65001
  2017-07-05T10:37:11.2234637Z Attempting to pack file: d:\a\1\s\src\Esoterix\Esoterix.csproj
  2017-07-05T10:37:11.2294640Z [command]d:\a\_tool\NuGet\4.0.0\x64\nuget.exe pack d:\a\1\s\src\Esoterix\Esoterix.csproj -NonInteractive -OutputDirectory d:\a\1\a -Properties Configuration=release -version 1.0.0-CI-201765-103711 -Verbosity Detailed
  2017-07-05T10:37:12.3087180Z NuGet Version: 4.0.0.2283
  2017-07-05T10:37:12.3187144Z Attempting to build package from 'Esoterix.csproj'.
  2017-07-05T10:37:12.7569409Z MSBuild auto-detection: using msbuild version '15.1.1012.6693' from 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\bin'. Use option -MSBuildVersion to force nuget to use a specific version of MSBuild.
  2017-07-05T10:37:15.3118046Z Packing files from 'd:\a\1\s\src\Esoterix\bin\release\netcoreapp1.1'.
  2017-07-05T10:37:15.7119285Z Add file 'd:\a\1\s\src\Esoterix\bin\release\netcoreapp1.1\Esoterix.dll' to package as 'lib\netcoreapp1.1\Esoterix.dll'
  2017-07-05T10:37:15.7959331Z System.InvalidCastException: Unable to cast object of type 'System.String' to type 'NuGet.Frameworks.NuGetFramework'.
  2017-07-05T10:37:15.7969309Z    at NuGet.ProjectManagement.NuGetProject.GetMetadata[T](String key)
  2017-07-05T10:37:15.7969309Z    at NuGet.ProjectManagement.PackagesConfigNuGetProject..ctor(String folderPath, Dictionary`2 metadata)
  2017-07-05T10:37:15.7969309Z    at CallSite.Target(Closure , CallSite , Type , Object , Dictionary`2 )
  2017-07-05T10:37:15.7969309Z    at System.Dynamic.UpdateDelegates.UpdateAndExecute3[T0,T1,T2,TRet](CallSite site, T0 arg0, T1 arg1, T2 arg2)
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.ProjectFactory.AddDependencies(Dictionary`2 packagesAndDependencies)
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.ProjectFactory.ProcessDependencies(PackageBuilder builder)
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.ProjectFactory.CreateBuilder(String basePath, NuGetVersion version, String suffix, Boolean buildIfNeeded, PackageBuilder builder)
  2017-07-05T10:37:15.7969309Z    at NuGet.Commands.PackCommandRunner.BuildFromProjectFile(String path)
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.PackCommand.ExecuteCommand()
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.Command.ExecuteCommandAsync()
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.Command.Execute()
  2017-07-05T10:37:15.7969309Z    at NuGet.CommandLine.Program.MainCore(String workingDirectory, String[] args)
  2017-07-05T10:37:15.8259323Z ##[error]Error: d:\a\_tool\NuGet\4.0.0\x64\nuget.exe failed with return code: 1
  2017-07-05T10:37:15.8259323Z ##[error]An error ocurred while trying to pack the files.
  2017-07-05T10:37:15.8289336Z ##[section]Finishing: NuGet pack
  </code></pre>
  
  <p>It seems that everything is picked up fine up to this point so I'm a bit stuck . Any ideas welcome.</p>