Can't build after upgrade to Xamarin.iOS 6.4

I get instances of this error:

The type System.Action' is defined in an assembly that is not referenced. Consider adding a reference to assemblySystem.Core, Version=3.5.0.0, ...

Under the Project menu, the Active Runtime is Mono 3.2.0.

When I bring up Edit References, System.Core shows 2.0.5.0, which seems wrong, but I'm not sure.

If I switch the Active Runtime to 2.10.12, I get different errors:

Cannot convert lambda expression' to non-delegate typeSystem.Action' ...

I should be able to build code using System.Action under the 3.2 runtime, right?

And it appears that the 6.4 upgrade somehow breaks support for using the old runtime?

Suggestions?