Error On someone elses' pc

Wrote a client server app. VB front end SQL back end.
Works fine on my PC.

On other users PC...I have a combo box to select a customer.

So once a customer is selected it is supposed to fill a site combo box which relates a site to a customer. So the click event of the customer is:

VB Code:

Private Sub cboCustomers_Click()
On Error GoTo Err_Handler
'function fills all the site names for the customer
Call FillSiteCBO
 
Done:
Exit Sub
 
Err_Handler:
MsgBox Err.Description, vbCritical, "Error #: " & Err.Number
Resume Done
 
End Sub

And FillSiteCBO looks like this:

VB Code:

Private Sub FillSiteCBO()
On Error GoTo Err_Handler
 
Dim objRSFill As ADODB.Recordset
 
Call EstablishConnection
Set objCmd = New ADODB.Command
Me.cboSites.Clear
objConn.CursorLocation = adUseClient
 
    With objCmd
        .ActiveConnection = objConn
        .CommandText = "select_sites_by_customer"       'our stored procedure is good_login
        .CommandType = adCmdStoredProc          'its a stored procedure
        .Parameters.Append .CreateParameter("CustomerID", adBigInt, adParamInput, , cboCustomers.ItemData(cboCustomers.ListIndex))
        Set objRSFill = .Execute
        Set objRSFill.ActiveConnection = Nothing
        Call ReleaseConnection
    End With
 
    If objRSFill.BOF Then
        'do nothing
    Else
        Do Until objRSFill.EOF
            cboSites.AddItem objRSFill.Fields("Name").Value
            cboSites.ItemData(cboSites.NewIndex) = objRSFill.Fields("SiteID").Value
            objRSFill.MoveNext
        Loop
    End If
      
Done:
Set objCmd = Nothing
Set objRSFill = Nothing
Exit Sub
 
Err_Handler:
MsgBox Err.Description, vbCritical, "Error #: " & Err.Number
Resume Done
 
End Sub

BUT every time another user besides myself runs this code they get:

Error # -2147217872

Type name is invalid

I however do not get this error.

Can anyone explain what the dilly is here?

Thanks,
Jon