Submit followed by error message

Hi,

When I submit my form at:

http://creator.zoho.com/three3k/form/4/wz48QOQ665mHv64gAbwT6Z72u6JsUqbMOKVUtxarJ5Kjr8mVTKGkNqymWG99uFJj90X0GtR1snsw97MABCa1SK2t0Emsvp8MQ5bZ/

I get the following error message (see also the attached screenshot):

Error in file upload :TypeError: document.getElementById(_4d[1]) has no properties

I don't use a file upload field. After clicking away the error message, the form appears to submit fine. Can anyone help me solve this?

Thanks!
Regards,
Diederik van Leeuwen
Telematica Instituut
The Netherlands