代码契约问题

考虑以下代码:

public class RandomClass
{        
    private readonly string randomString;

    public RandomClass(string randomParameter)
    {
        Contract.Requires(randomParameter != null);
        Contract.Ensures(this.randomString != null);

        this.randomString = randomParameter;
    }

    public string RandomMethod()
    {
        return  // CodeContracts: requires unproven: replacement != null
            Regex.Replace(string.Empty, string.Empty, this.randomString);
    }
}

这确保了 randomString RandomMethod 被执行。为什么代码契约分析会忽略这一事实并抛出 CodeContracts: requires unproven: replacement != null 警告?