I'm in the process of writing an excel sheet to do a series of calculations
and lookups etc. and have got to the final calculation and cant make the step
work!

I need to calculate the reciprocal of 6 numbers. i.e 1/a = 1/b+1/c+1/d etc.
fairly standard, but i dont always have a value and i want to make excel
ignore the step if there is no value in a particular cell (these are linked
from previous sheets and currently have =if(cellx = "yes", celly,"")).
However, when the "" is used for 1/b+1/c etc. i get a #value error.

Any ideas on ways around this would be greatly appreciated.