NCover 1.5.8 - VerificationException

When I run certain tests under NCover I get a security exception, e.g.

TestCase 'Winona.Emailer.Core.Test.PropertyMergerFixture.NoVariables'

failed: System.Security.VerificationException : Operation could destabilize the runtime.

at Winona.Emailer.Core.PropertyMerger..ctor(IDictionary`2 properties)

C:\Devel\Winona\code\Core\Test\PropertyMergerFixture.cs(29,0): at Winona.Emailer.Core.Test.PropertyMergerFixture.Setup()

If the same tests are run under NUnit, they run correctly - is this caused by the injection process required for profiling the lines?

Regards

Paul