ExecutionEngineException on iOS only ??

I'm running some custom networking code (not sure if that's related or not), but said networking code makes use of the C# Interlocked functionality. The following exception is only thrown when running the app on iOS hardware (IPad 2, 5.0)

... it works flawlessly on Windows and on Mac.

Any idea why this is blowing up on iOS?

ExecutionEngineException: Attempting to JIT compile method '(wrapper managed-to-native) System.Threading.Interlocked:CompareExchange...

ExecutionEngineException on iOS only ??