ISERROR Formula

[Dibbley247]

This formula wont display a result, and will have #VALUE when other boxes are clear. I have similar formula working everywhere else, except this one?
I want the cells to look blank when there is nothing to add.

this is what i'm trying.

=IF(ISERROR(T6*$Z$7)+(U6*$Z$7),"",(T6*$Z$7)+(U6*$Z$7))

This is the original formula:

=(T6*$Z$7)+(U6*$Z$7)

Thanks