Link Tables: #Num! Due to Blank Cells

Hi All - newbie alert so please be gentle! :D

I've got fairly solid excel skills but am new to access so this is probably why i'm having difficulty with this! I'm on Access 2007-10.

I've found numerous references online to #Num! in link tables but none that seem to match my situation and i don't know access well enough to expand the logic to solve my problems.

I have link table in my db which has a two columns that primarily contain a date for each row (format in dd/mm/yyyy hh:mm). However, there are a number of rows which are intentionally blank but in reality contain a space (" "). When i open the link table these cells now contain #Num!.

I am trying to replicate the excel formula which i use: =IF(a2=" ","SAF","Sold")

In access i have therefore tried:

= IIF([Payment Received Date]=" ","SAF","Sold")

but because of the #Num! that just gives the correct "Sold" where there is a date but #Num! where there isn't one.

I have also tried:

= IIF(IsDate([Payment Received Date]),"Sold","SAF") and,

= IIF(IsError([Payment Received Date]),"SAF","Sold")

both get stuck on the #Num! still.

Any ideas? Can i fix the link file or is there an alternative formula i can try?

Thanks
Bob.