Runtime error: The assembly mscorlib.dll was not found or could not be loaded

<Warning>: The assembly mscorlib.dll was not found or could not be loaded.
<Warning>: It should have been installed in the `/Developer/MonoTouch/Source/monotouch/builds/install/target64/lib/mono/2.0/mscorlib.dll' directory.
<Warning>: Service exited with abnormal code: 1

This issue occurs when the hidden .monotouch-32 and .monotouch-64 folders are missing from the .xcarchive for signing / IPA creation, triggering the runtime error.