System.Threading.SpinLock not blocking

How is it possible that a System.Threading.SynchronizationLockException ("The calling thread does not hold the lock") is thrown at the _lockObject.Exit() call when lockAcquired is true and no other threads have even attempted to get the lock?

public sealed class SomeClass
{
    private readonly SpinLock _lockObject = new SpinLock(true);

    public void SomeMethod() 
    {
        bool lockAcquired = false ;
        try
        {
            _lockObject.Enter(ref lockAcquired);
            // write some text to a log file.
        }
        finally
        {
             if (lockAcquired) _lockObject.Exit();
        }
    }