Cannot chaneg sort order on float field

[Salbrox]

I have a query with a float/delta column which is the expression:

 Float:  [Date1] - [Date2]

When I try to change the order by in the column filter drop-down i get a data type mismatch in criteria expression error.

Does anybody know why I might be getting this error?

Thanks in advance for your help.

 

[plog]

Not every record is resolving to a value because one of the fields contains an unexpected data type. For example, Date1 might be a string and Date2 might be a number which makes this numerical evaluation unresolvable:

10/1/2013 - 10/31/201B

If you're familiar with Excel, that evaluation would return the equivalent of #N/a. Then when you go to sort on that column it has no idea what you are talking about because that value isn't sortable.

I'd run a query using the IsDate function (http://www.techonthenet.com/access/functions/advanced/isdate.php) to make sure every value in Date1 and Date2 is a date value (assuming they are supposed to be dates).