I am receiving weird error while attempting Test Callout (#5), it is giving below error.

Challenge Not yet complete... here's what's wrong: 
The 'WarehouseCalloutServiceTest' class does not appear to be using the 'WarehouseCalloutServiceMock' mock callout class.

But my WarehouseCalloutServiceTest is calling the mock class and it shows 100% code coverage for WarehouseCalloutServiceMock class after running the test. 100% coreage for mock class shows that I am using the class. So,  Can anyone help in figuring what can be the issue.