Is there a way to have a formula in numbers 09 return an empty value (ie isempty() returns true?)

Question

I've got a list of times and ages, and want to extract ages for a particular range of time values. So I've used an 'if' formula to copy the age for an entry if its time is between two values. The problem is the value for entries outside that time range. I'd like value to be 'empty' or 'null'. I've tried including no argument in the 'if' formula and also and empty string (""). A check with 'isempty' returns true. For example:

=WENN(UND($D>=F$1;$D<G$1);$C;)
=WENN(UND($D>=F$1;$D<G$1);$C;"")

both of these produce values that aren't 'empty'.