invalid operation

[solar]

CaN somebody help me find the error in my code ? I get the error
"invalid operation" with the following line :

tdf.Fields("CustomerID") = 121

Public Function Test()
Dim dbs As DAO.Database
Dim rst As DAO.Recordset
Dim fld As DAO.Field
Dim tdf As DAO.TableDef

Set dbs = CurrentDb
Set rst = dbs.OpenRecordset("orders1", dbOpenDynaset)
Do While Not rst.EOF

rst.MoveNext
Set tdf = dbs.TableDefs("Orders1")

Set fld = tdf.Fields("CustomerID')
tdf.Fields("CustomerID") = 121

Loop

rst.Close
Set rst = Nothing
Set dbs = Nothing
End Function

[pietlinden]

try
Do While Not rst.EOF

rst.Edit
rst.Fields("CustomerID")=121
rst.Update
rst.MoveNext

Loop