Why Am I getting a blank in the result cell?

[salulu]

Can someone assist me with what I'm doing wrong with the formula below?
It keeps returning a blank in the resulting cell. I am trying to
calculate the weighted avg. of production scores for an employee based
on hours worked per month. columns B,D,F,H,J, and L are the hours while
C,E,G,I,K, and M are the prod scores. Manual calculation should result
in a 90.9% weighted averge.

B C D E F G H I J K
L M wgtd. Avg.
129 117 - - 29 75 19 32 - - 50
55 90.9%

=IF(ISERROR(SUMPRODUCT((B13*C13)+(D13*E13)+(F13*G13)+(H13*I13)+(J13*K13)+(L13*M13))/(B13+D13+F13+H13+J13+L13)),"",SUMPRODUCT((B13*C13)+(D13*E13)+(F13*G13)+(H13*I13)+(J13*K13)+(L13*M13))/(B13+D13+F13+H13+J13+L13))

Note: Columns D,E and J,K are blanks. Also, the data above are linked
from other worksheets with "IF" statements.

Whoever can solve this would be be my HERO ........

 

[cvolkert]

I tried the formula and it seems to work alright for me. My hunch i
that in D,E,J or K you have a space instead of a blank. To be safe
if you type a zero in each of these 4 cells that are supposed to b
blank, your formula should return 90.9% Let me know if this doesn'
work - Cha

 

[duane]

it sounds like you have text in some of the cells in the denominato
which results in the error defaul

 

[cvolkert]

I tried the formula and it seems to work alright for me. My hunch i
that in D,E,J or K you have a space instead of a blank. To be safe
if you type a zero in each of these 4 cells that are supposed to b
blank, your formula should return 90.9% Let me know if this doesn'
work - Cha

 

[salulu]

Thanx to both chad and duane. I solved the problem. I took part of
chad's advise by replacing the " " in my IF statement with "0s" in the
linked data cells, and whoola, it worked. You guys are my hero
..............