Exception when headers are numbers

[sentry-io]

Sentry Issue: COVE-FG

AttributeError: 'float' object has no attribute 'startswith'
(2 additional frame(s) were not displayed)
...
  File "__init__.py", line 244, in unflatten
    with_heading_source_map=heading_source_map,
  File "flattentool/input.py", line 360, in fancy_unflatten
    cell_tree = self.do_unflatten()
  File "flattentool/input.py", line 310, in do_unflatten
    for j, line in enumerate(lines):
  File "flattentool/input.py", line 182, in convert_dict_titles
    for d in dicts:
  File "flattentool/input.py", line 617, in get_sheet_lines
    elif sheet_configuration.get("hashcomments") and header.startswith('#'):

'float' object has no attribute 'startswith'

[Bjwebb]

cove error: 'float' object has no attribute 'startswith'