Field Index Out of Range error

I'm trying to programatically create a Master-Detail relationship
between a pair of dBase III tables using the following code:

   Orderstable.MasterSource := CustomerDataSource;
   Orderstable.MasterFields := 'CUST_NO';

When I execute this code, I receive an error message saying:

   Orderstable: Field Index out of range.

The data in the CUST_NO field is valid and complete in both tables.
Can someone tell me what is causing this error and how to correct it?
Thanks very much.

Bill