[galaxy-dev] Optional column select arguments?

Dear all, I'm trying to define an *optional* column parameter as follows: <param name="start_col" type="data_column" data_ref="input_tabular" multiple="False" numerical="False" optional="True" label="Column containing start co-ordindate" help="Leave empty to include everything from the first base/residue"/> i.e. I want the user to either select ONE of the columns of the input tabular data file (in which case getting "1" or "c1" etc as the string is fine) OR pick no column at all (in which case I want an empty string or similar in the command string. Is this possible? The above parameter loads fine, but defaults to the first column with no apparent way to un-make this implicit selection. Thanks, Peter