Hi, I have been trying the "Using expressions challenge" of the lightning module and when I check the challenge I receive the next message:

Challenge not yet complete... here's what's wrong: 
The component is not defining an attribute

And here is my component:
<aura:component implements="force:appHostable">
    <aura:attribute name="DayOfTheWeek" type="String" default="Monday" /> 
    <aura:if isTrue="{!v.DayOfTheWeek=='Monday'}">
    Today is {!v.DayOfTheWeek}
        .........
    </aura:if>
</aura:component>

So, basically I don´t know what the error message refers to.

Thanks in advance.