Entity Framework Unit Testing : Operation not supported. Unknown error: 0x80070057

I'm getting exception while doing unit testing (Framework: Microsoft.VisualStudio).

Here is My Unit Test Code is:

    [TestMethod]
    public void Add_FlatRateWithCorrectEntityTest()
    {
         FlatRateInfo rate = new FlatRateInfo()
            {
                Code = "JB127",
                VDSCode = "Wfgv7",
                FlatRate = "200.00"
            };
            var response = _flatRateService.AddAllFlatRate(rate);            
            Assert.AreEqual(FlatRateResult.Success, response);            
    }

Here, I am getting error:

var jobDetails = UnitOfWork.JobRepository.GetSingleOrDefault(s => s.JobCode == flatRate.Code);
if(jobDetails != null)
{
// My logic
}