SSRS Parse Date Error

Hi, 

Any idea why getting an #Error using the code below when the field(fp_firstshifttimein) does not contain value. But when the the field contains value, it works.

=IIF(IsDate(Fields!fp_firstshifttimein.Value), DateTime.Parse(Fields!fp_firstshifttimein.Value).addDays(8), Nothing)

Same issue with the code below:

=IIF(IsNothing(Fields!fp_firstshifttimein.Value), Nothing, DateTime.Parse(Fields!fp_firstshifttimein.Value).addDays(8))

Thanks all.