Why is Code Contracts suggesting that I require a parameter to be null?

Question

Looking at the code sample below, Microsoft Code Contracts warns:

CodeContracts: Missing precondition in an extremely visible method. Consider adding Contract.Requires(science == null); for parameter validation

Am I missing something here? Why would CC suggest that I require the parameter to be null, which is the exact opposite of what should be done here?

I'm using VS2015, .NET 4.6.

using System;
using System.Diagnostics.Contracts;
public sealed class Weird
{

    public Weird(object science)
    {
        if (null == science)
        {
            throw new ArgumentNullException();
        }

        Contract.EndContractBlock();

        this.Science = science;
    }
    private object Science { get; }

    [ContractInvariantMethod]
    private void ObjectInvariant()
    {
        Contract.Invariant(null != this.Science);
    }
}

Answer 1

This isn't a much better answer than your workaround, but I got it to "work" by simply changing:

private object Science { get; }

to:

private object Science { get; set; }

Basically the same as your workaround but without providing your own backing field. I guess Code Contracts doesn't quite understand the new syntax.