Excel True/False Field with Nulls displaying as False

Using CRXI with Excel spreadsheet as my database. The Excel file has a field with entries of either True, False, or Null (blank). When displaying this field in CR, the Null values are displaying as False. How do I display the Nulls as blank. I tried the following formula, but the Nulls still are False:

If(IsNull(field)) then ""

else ToText(field)

Thanks,

JSR