CrossTab Query and report Field Errors

[mearmortal]

Using a Query with the following: -

Week: Format([TheDate],"ww&quot /* Column Heading
Value: 1-([SumOfFailed]/[SumOfTested]) /* Value

Depending on the informaton some week numbers say from 23 to 52 don't exist right now, how do I stop the report from failing because of invalid field names.
Appart from removing the Fields of course.

Each of the Fields call up the following in the controlsource: -

=IIf([1]>0,[1],1) through =IIF([52]>0,[52],1)

is their something else I need to set.