Failed to resolve assembly: System.Runtime after adding Flow Canvas package

I posted this over on the flow canvas forum but might have more luck here.

After I add the flow canvas package from the Unity store to a project and try to build for Windows Store I get this error:
AssemblyResolutionException: Failed to resolve assembly: 'System.Runtime, Version=4.0.20.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
Mono.Cecil.BaseAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name, Mono.Cecil.ReaderParameters parameters)
Mono.Cecil.BaseAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name)
Mono.Cecil.DefaultAssemblyResolver.Resolve (Mono.Cecil.AssemblyNameReference name)
Mono.Cecil.MetadataResolver.Resolve (Mono.Cecil.TypeReference type)
Mono.Cecil.ModuleDefinition.Resolve (Mono.Cecil.TypeReference type)
Mono.Cecil.TypeReference.Resolve ()
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:192)
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:194)
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:194)
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:194)
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:194)
UnityEditor.AssemblyReferenceChecker.InheritsFromMonoBehaviour (Mono.Cecil.TypeReference type) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:194)
UnityEditor.AssemblyReferenceChecker.MethodIsMouseEvent (Mono.Cecil.MethodDefinition method) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:177)
UnityEditor.AssemblyReferenceChecker.CollectReferencedAndDefinedMethods (Mono.Cecil.TypeDefinition type, Boolean isSystem) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:156)
UnityEditor.AssemblyReferenceChecker.CollectReferencedAndDefinedMethods (IEnumerable`1 assemblyDefinitions) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:120)
UnityEditor.AssemblyReferenceChecker.CollectReferences (System.String path, Boolean collectMethods, Single progressValue, Boolean ignoreSystemDlls) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:111)
UnityEditor.AssemblyReferenceChecker.GetScriptsHaveMouseEvents (System.String path) (at C:/buildslave/unity/build/Editor/Mono/Utils/AssemblyReferenceChecker.cs:276)
UnityEditor.HostView:OnGUI()

I've tried manually adding a reference to System.Runtime, Version=4.0.20.0 to the VS project but doesn't make any difference.

I'm targeting Win 10 and .Net scripting backend. Project compiles fine to a exe if I switch build targets.

In ILSpy I did notice these differences:
Assembly-CSharp.dll
// System.Runtime, Version=4.0.20.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
[assembly: TargetFramework(".NETCore,Version=v5.0", FrameworkDisplayName = ".NET for Windows Universal")]

nunit.framework.dll
// System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
[assembly: TargetFramework(".NETPortable,Version=v4.5,Profile=Profile259", FrameworkDisplayName = ".NET Portable Subset")]

Do I maybe need a specific Windows SDK version?

Any help is appreciated!

 

Thanks! Using 5.6.0f3. Is there a .Net workaround if in the off chance I cant use 5.6.0p1?

Also from my thread in the Flow Canvas forum, there's a "fix":
http://flowcanvas.paradoxnotion.com/topic/error-building-windows-store-app/#post-1420