Iif Problem Brings Back #Error

I'm pulling a "Number (8)" data type field for a date (formatted YYYYMMDD) from a Oracle 9i database. When the value is inserted into the database it is set a zero (A non-null database for the most part), otherwise it is for example 20061224. If the date field has a value the date is displayed correctly, but if it is zero then I get the "#Error" message.

Here is one of many iif expressions I've tried.

=iif(Len(CStr(Fields!DTE_MAILED.Value)) = 8,

((CStr(Fields!DTE_MAILED.Value)).Substring(4,2) + "/" + Right(CStr(Fields!DTE_MAILED.Value), 2) + "/" + Left(CStr(Fields!DTE_MAILED.Value), 4))

,

Nothing)

Now I've returned the value with out any formatting done to the string, and it will return "0"(zero) or a number. I've returned the lengths of the returning value and it comes back "1" or "8". I read a lot of previous posts and I thought at first that it was because I was trying to do a substring function on the zero value getting a index error. So I've changed the iif test condition many different ways with no prevail. I read a previous post where someone ended up doing his work in his SQL, but I would like to find out how to do this in the report. This is a simple expression, so I feel like there is something obvious I don't know maybe something with the format mask.

All help will be appreciated!