I was getting the same error until I switched to a standard If statement as follows.

MyEmailBody = myDataRow("AccountId") & " " & _
myDataRow("OrderNumber")
IF Not IsDBNull(myDataRow("APN")) THEN
myDataRow &= " " & Trim(myDataRow("APN")))
END IF
IF Not IsDBNull(myDataRow("StreetNumber")) THEN
myDataRow &= " " & Trim(myDataRow("StreetNumber")))
END IF

Not sure why the IIF acts the way it does - Would like to find out