NAN retunred on calculating a figure

Morning All,

I am trying to create a calculation column  but for some rows i get a NAN.

I think this is happening when the figures i am trying to calculate from contain a NULL value.

Column i am trying to achieve is   Debit  - Credit =  Net

Debit is calculated using code :-  <?xdofx:nvl ((JRNL_LINE_DR[.!='']),0)?>

Credit is calculated using code :-  <?xdofx:nvl ((JRNL_LINE_CR[.!='']),0)?>

Net is  :-  <?choose:?><?(JRNL_LINE_CR - JRNL_LINE_DR).!=''''?><?format-number:(JRNL_LINE_CR - JRNL_LINE_DR).!='''';'999G990D99'?><?otherwise:?>0.00<?end otherwise?><?end choose?>