This really Seems like a data type mismatch issue.
Are TestNumMois values like 05,06,07? They should be 5,6,8.
There is no problem with your expression the field TestNumMois i think is coming in as a text value, that's why it won't evaluate.
Try this to verify please; in a textbox = IsNum(TestNumMois) ; if this returns false then TestNumMois is a text value and you will need to convert this to Num while loading.