AttributeError: Name Conflict in fieldset `Plate Description`. The name(s) `['i']` appear multiple times.

[hugokitano]

class NewSamplesheetForm(BetterForm):
    class Meta:
        fieldsets = (
            ('', {'fields':('experiment', 'plate_id')}),
            ('Plate Description', {'fields': ('description')}),
            ('Nanoparticle', {'fields':('np_vol_ul', 'nanoparticle_diluent')}),
            ('Biosample', {'fields':(('biosample_id', 'biosample_type'), ('biosample_dilution_factor', 'biosample_diluent'))}),
            ('Assay', {'fields':(('assay_instrument', 'cleanup_instrument'), 'assay_wash_buffer', ('incubation_time_hr', 'incubation_temp_c'),
                    'digestion_method', 'sample_vol_ul')})
        )

When I do this, I get the above error message. It seems to be looping over the letters of "description", thus getting two "i's" rather than looping through each field.