Error 'Too few parameters. Expected 1.'

Hi, excuse the cross posting. I did post this in Forms Coding, but didn't
get 
a response so I think I may have posted in the wrong place (seeing as my 
error occurs when using ADO). My apologies, I wouldn't normally post
twice! 
Anyway I've got a strange problem.

I've got a form 'EditPartNumbers' where a user can filter the records by 
selecting a value in a combo box, which in turn populates a value in a
hidden 
text box, 'txtFindHidden'. The form is then supposed to change it's 
datasource to a query 'PartNumbersFine_EditForm', which has a parameter in

it's RecordID field which is [Forms]![EditPartNumber]![txtFindHidden].

If I select a record from the combo box and run the query directly from 
Access it works fine, but if I try and run it through the following code,
I 
get an error:

'Run-time error -2147217904 (80040e10): Too few parameters. Expected 1.'

Here's the code:

 Dim db As ADODB.Connection
    Dim rs As New ADODB.Recordset
    Set db = CurrentProject.Connection
    
    'firstly get a recordset to check if there are any records
    If formMode = "New" Then
        rs.Open "PartNumbersNew_EditForm", db, adOpenKeyset, 
adLockOptimistic, adCmdTableDirect
    Else
        'if the form isn't showing just new parts then get the recordset 
depending on whether it's a find part or not
        If findPart = "" Then
            rs.Open "PartNumbersAll_EditForm", db, adOpenKeyset, 
adLockOptimistic, adCmdTableDirect
        Else
'this is where it stalls!            
rs.Open "PartNumbersFind_EditForm", db, adOpenKeyset, adLockOptimistic, 
adCmdTableDirect
        End If
    End If

I've searched and it would seem that the most common problem is a syntax 
one, but I can't understand that it in this case.

Any ideas?

Thanks
Julia