Very confusing .... I've been using a pretty basic PowerShell DSC custom resource that deals with windows features. It's been working as expected for quite some time now, I thought, but I ran across some machines that appear to have an issue with the key.
When I use nullable[string] as the type, it works for almost all machines, but some throw a "Value cannot be null" error when attempting to import the resource. Removing "nullable" from the property type and leaving just as a string fixes the issue. These are all Server 2016, PS v5.1.14393.3053, .NET 4.6.01586, installed windows updates all are identical.