pandas to_excel() using float_format parameter --> ValueError: could not convert string to float

I have a pandas DataFrame with heterogenous data. That means that some columns are floats, some are strings, etc.

I first tried formatting the columns by calling the xlsxwriter worksheet-level set_column() method, but it appears that to_excel() is formatting each individual cell with it's own format object, so the column-level format is getting overridden.

I'm trying to export a DataFrame to Excel and take advantage of the float_format parameter documented here.

The code:

writer = pd.ExcelWriter(path, engine='xlsxwriter')

ff = '_(* #,##0.00_);_(* (#,##0.00);_(* "-"??_);_(@_)'

df.to_excel(writer, "sheet_name", index=False, float_format=ff)

The Exception I'm getting upon calling to_excel:

$VE_DIR/lib/python2.7/site-packages/pandas/util/decorators.pyc in wrapper(*args, **kwargs)
     58                 else:
     59                     kwargs[new_arg_name] = old_arg_value
---> 60             return func(*args, **kwargs)
     61         return wrapper
     62     return _deprecate_kwarg

$VE_DIR/lib/python2.7/site-packages/pandas/core/frame.pyc in to_excel(self, excel_writer, sheet_name, na_rep, float_format, columns, header, index, index_label, startrow, startcol, engine, merge_cells, encoding, inf_rep)
   1228         formatted_cells = formatter.get_formatted_cells()
   1229         excel_writer.write_cells(formatted_cells, sheet_name,
-> 1230                                  startrow=startrow, startcol=startcol)
   1231         if need_save:
   1232             excel_writer.save()

$VE_DIR/lib/python2.7/site-packages/pandas/io/excel.pyc in write_cells(self, cells, sheet_name, startrow, startcol)
    785         style_dict = {}
    786 
--> 787         for cell in cells:
    788             num_format_str = None
    789             if isinstance(cell.val, datetime.datetime):

$VE_DIR/lib/python2.7/site-packages/pandas/core/format.pyc in get_formatted_cells(self)
   1729         for cell in itertools.chain(self._format_header(),
   1730                                     self._format_body()):
-> 1731             cell.val = self._format_value(cell.val)
   1732             yield cell
   1733 

$VE_DIR/lib/python2.7/site-packages/pandas/core/format.pyc in _format_value(self, val)
   1510                 val = self.inf_rep
   1511             elif self.float_format is not None:
-> 1512                 val = float(self.float_format % val)
   1513         return val
   1514 

ValueError: could not convert string to float: _(* #,##0.00_);_(* (#,##0.00);_(* "-"??_);_(@_)

I would assume to_excel() would only try to apply the parameter to float-formatted columns (or even specific cells) rather than to every piece of data, so I'm not sure what I'm missing. If need be I'll post a cleaned version of the specific table that reproduces the error, but I thought perhaps someone would recognize what I'm facing.

Thank you!