Wierd results for logical test on a blank cell

[masplin]

I have a field [ADDR12] with along winded formula that includes the following clause.

=IFERROR(
                                             ISNUMBER(
                                                                    mid( [Addr12],3,1)+0
                                                                  )
                                             , FALSE
                                              )

On one table this field returns FALSE which is what you would expect.

However i have another table that contains the same field (but it might be from a different table in the DB)
that also looks blank and =ISBLANK([ADDR12) = TRUE so it is real blank.

however the above formula in this case = TRUE!!!!!

i am completely mystified how a blank cell, confirmed as BLANK can return TRUE when tested as a number.

Can anyone enlighten me as the 2 cells look identical but I get opposite results.

Thanks
Mike