DeveloperBarn

Ok guys, this has me stumped. The title is the message I get for some of the queries I have written. I have a table with some dates and names in it. In particular I want to know all the applications that have been executed in the prior month.
I have an expression:
Expr1: IIf(Format([ndaDteEx],"mm")<10,Mid("00" & Format([ndaDteEx],"mm"),3) & Format([ndaDteEx],"yyyy"),Format([ndaDteEx],"mm") & Format([ndaDteEx],"yyyy"))
In the criteria:
IIf(Format(Date(),"mm")<10,Mid("00" & Format(Date(),"mm")-1,2) & Format(Date(),"yyyy"),Format(Date(),"mm")-1 & Format(Date(),"yyyy"))
There are several tables and I run a similar query against each one with only the field name changed appropriately. Some of the queries appear to run fine, and others give me the error message.

Your thoughts???
Alan

I found the answer. In setting up the db, I had left some empty fields that were actually populated with blanks ie. "" in some date fields. This resulted in the error message. Once the fields and records were deleted, the queries ran as designed.
Alan