Getting valid values for numeric grouping in Pivot Tables

[TomCon]

In a Pivot Table if you put a numeric column in the row area, you can rt. click on any value in the row area, select "group" from the pop-up menu, and Excel will offer you the numeric grouping dialog that allows you to select from/to/step for numeric grouping. But, if you have even one error value in that column, Excel just says "cannot group that selection".

So, i tried two things to overcome this. Neither worked. So, I wonder if anybody knows how I can get around this to be able to still group that numeric column by numeric ranges.

Here is what i tried. I used IFERROR in the formula in the source column to make the error value either "" or #NA . But neither solves the problem and allows numeric grouping of the column in a Pivot Table. These are the formulas.
=IFERROR([@TimeValue]-G1,"")
=IFERROR([@TimeValue]-G1,NA())

Does anybody know how i could still group this column numerically in a Pivot Table?

Thanks!