IIS v7.5 IRewriteProvider built using .NET 4.0 not found

I wrote a rewrite provider for IIS 7.5 in C#, .NET 4.0. When I try to have IIS manager locate it so that I can configure and use it for URL rewriting, IIS manager can't find it. When I compile it using .NET 3.5, everything works fine.

The library I b开发者_运维知识库uilt is in the .NET 4.0 GAC_MSIL folder.

I need to use .NET 4.0 because I'm pulling in another library that uses 4.0.

Any suggestions are appreciated,

mj