Filtering values in ResultSet by setting range on complex index

Hi!
I have table with complex index on 5 fields. One of them is string filed. I want to implement some sort of filtering, by setting SetRange() in my SQLCeCommand. But i need to fileter only by one string field and to get the values starting with the input string value.
I tried to use such code:
...
command.SetRange(DbRangeOptions.Prefix, new object[] {null, null, null, "Com", null}, null);
resultSet = cmd.ExecuteResultSet(ResultSetOptions.Scrollable);
....
But it doesn't work. As a result i've got an empty result set.
Usage of simple index on one field and setting the correspondent range will solve problem, but i can't have such index due to project restrictions.
Is there any way to set prefix range only by one value of complex index? If not, please, explain me how does Prefix Range works.

Thanx

If filtering has the same behavior as seeking (which I am assuming - read: not tested) then you might want to consider putting the column with the non-null value as the first in the index. This will require that you recreate the index with the new column order.