Data type mismatch in criteria expression

here's the error:

Microsoft JET Database Engine error '80040e07'

Data type mismatch in criteria expression.

here's the code:

 Dim strConn ' Database connection string
 Dim SQL  ' String that will have our SQL statments

 ' Time to create and open recordset
 Set RS = Server.CreateObject("ADODB.Recordset")
 RS.CursorLocation = 3 ' adUseClient
 RS.Open SQL, strConn ' adOpenKeyset CursorType

error is reported on that last line of code. Any ideas / help? Thanks.