I'm refamiliarising myself with Extension Classes and have set up the basic class framework (as I have before), but this time Intellisense isn't happy

Since the image may not be clear enough the statement it objects to is:
<IDOMethod(MethodFlags.None, "Infobar")>
and the error is:
Enum Mongoose.IDO.MethodFlags - BC32035: Attribute specifier is not a complete statement
I've double checked this against the syntax in a class I built some time ago (and which works) but can't see any differences. The only thing which differs between the two is that the new class has a target framework of .NET 4.7.2 whereas the working one was targeted at 4.6.1. (and rebuilds without error)
Can anyone advise what I'm doing wrong?