vlookup function referencing a formatted cell

[kgallo]

Hi,
I am trying to use the vlookup function but the range of reference has that little green triangle in the corner and if I f2 each cell that triangle goes away and the vlookup function works great. Do you know what is going on here? And also, a quick way to convert all the cells with the green triangle to be without the triangle in a whole range? I don't want to have to go to each cell individually and f2.
I've attached an example worksheet for reference to what I am describing.
vlookup format.xlsx
Thanks!

[zbor]

Your value in D column are not numbers.

Select empty cell
Select D column
Paste Spetial
Add
OK

[kgallo]

That definitely works! Thank you!