Is this a bug for walls food upgade?

[British Coffee]

My level 8 walls can use food to upgrade. Don't know any lower level walls but it seems like I can upgrade my lv8 walls with food even though the thing says I can use food from level 9

 

[Veldan]

I'm pretty sure it's not a bug... level 8 walls are the first ones that can upgrade with food (I have a couple of lvl 7 walls so can verify). They said level 9 yes, but I think the meaning of that is to level 9, not from level 9.

 

[British Coffee]

As long as they don't take down the upgrade, I'm happy with it.