End Google Ads 201810 - BS.net 01 --> Is it legal for a .NET 2.0 executable (.exe) to have a reference to a .NET 4.0 library (.dll)?

When I try this, the compiler complains that it can't resolve any of the types that come from the library.

As soon as I change the exe to .NET 4.0, the errors go away and it compiles cleanly.

Any thoughts? Thanks.