Access2000 Nested IIf() Functions In Date/time Format

I am working on a report in Access2000 which is getting information from several tables via a number of queries.
I am using nested IIf() functions to combine text formatted data in a calculated query field before using it in a report.

One such IIf() function is

Desc_col: IIf([HOVR - UDP].[SERVICE] Is Not Null,[DA] & " " & [SEQ] & " " & [TYPE] & [MO] & " " & [SUB] & " - " & [HOVR - UDP].[SERVICE],
IIf([MinOfCCT] Is Not Null,"I/O DIST CAB " & [UDP - JC CIRCUITS-05-F].[PWR] & " - CCTS " & [MinOfCCT] & " thru " & [MaxOfCCT],IIf([DESCRIPTIO] Is Not Null,[DESCRIPTIO],"Spare")))

When I have tried to do the same thing with date fields it does not work. The function is fine as long as it is not nested.

That is

Rev_col: IIf([Rev_HOVR] Is Not Null,[Rev_HOVR],"")

and

Rev_col: IIf([Rev_Elec_equip] Is Not Null,[Rev_Elec_equip],"")

both work.

But when I combine them into a nested function

Rev_col: IIf([Rev_HOVR] Is Not Null,[Rev_HOVR],IIf([Rev_Elec_equip] Is Not Null,[Rev_Elec_equip],""))

I get an error message "! Data type mismatch in criterial expression"

There is no data mismatch in the original tables from which the information was extracted. The dates are all in DATE/TIME and set to "General"format in the tables.

I am wondering if I will have to use Visual Basic function to combine these fields. However that poses a problem. When I've looked at this I find that runtime VB is no longer available in Access and I do not want to go out and buy it on the offchance that it will work.

Can anyone tell me if there is another solution to my problem?