reporting services - Why does SSRS start displaying #Error in a field when I change page, but works fine initially? -

odd one.

i have report returns list of staff absences - there's row per continuous period of absence listing dates, number of days absence lasted , column displays number of absences.

the report works fine displaying expect, below:

however, if change second page occasions column stops displaying figures , displays #error instead. if change page 1 continues show #error though working when report rendered:

likewise, if export excel displays #value instead of data. occasions column =code.getnumabsences(fields!staff_code.value). custom code it's calling is...

public function getnumabsences(byval staffcode string) integer      return cint(mytable(staffcode)(3))   end function 

mytable hashtable string (staffcode) key, , 3rd item integer. have no idea why report throw error after changing page when works fine @ runtime. can shed light on this?