ORA-01036: illegal variable name/number

Hi,
I'm using formdetails to add/update data.
On one page, it is working fine, on another I keep getting the error 1036 (ORA-01036: illegal variable name/number) when I try to update a record.

I've taken the update statement down to as basic a statement as possible in order to identify the problem and have used sqlmonitor to see what is being passed to Oracle.

Below is what I get in the trace.
I just don't see what the problem can be.

sam_account_name is varchar(30)

-----------------------------------
Timestamp: 09:18:53.816
UPDATE t_users SET sam_account_name = :sam_account_name WHERE sam_account_name = :sam_account_name
sam_account_name = 'm'
Execution time: 0 ms

Runtime error occurred: 1036 (ORA-01036: illegal variable name/number)
-----------------------------------

this is the asp definition
    <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConflictDetection="CompareAllValues"
        ConnectionString="<%$ ConnectionStrings:Tempus %>" DeleteCommand="DELETE FROM t_users WHERE user_id = :original_user_id "
        InsertCommand="INSERT INTO t_users (user_id, sam_account_name, username, department_id, company_id, role_id) VALUES (seq_t_users.nextval, :sam_account_name, :username, :department_id, :company_id, :role_id)"
        OldValuesParameterFormatString="original_{0}" SelectCommand="SELECT * FROM t_users"
        UpdateCommand="UPDATE t_users SET sam_account_name = :sam_account_name WHERE sam_account_name = :sam_account_name "
        ProviderName="<%$ ConnectionStrings:Tempus.ProviderName %>">
        <DeleteParameters>
            <asp:Parameter Name="original_user_id" Type="Int32" />
        </DeleteParameters>
        <UpdateParameters>
            <asp:Parameter Name="sam_account_name" />
            <asp:Parameter Name="username" />
            <asp:Parameter Name="department_id" />
            <asp:Parameter Name="company_id"  />
            <asp:Parameter Name="role_id" />
            <asp:Parameter Name="original_user_id" Type="Int32" />
            <asp:ProfileParameter Name="userID" PropertyName="userID" Type="Decimal" />
        </UpdateParameters>
        <InsertParameters>
            <asp:Parameter Name="sam_account_name" Type="Empty" />
            <asp:Parameter Name="username" Type="Empty" />
            <asp:Parameter Name="department_id" Type="Empty" />
            <asp:Parameter Name="company_id" Type="Empty" />
            <asp:Parameter Name="role_id" Type="Empty" />
            <asp:ProfileParameter Name="userID" PropertyName="userID" Type="Decimal" />
        </InsertParameters>
    </asp:SqlDataSource>

thx
Graham