dcc.Upload... "TypeError: 'float' object is not iterable"

I’m not entirely sure why I’m getting this error.

Basically I have two upload buttons that returns stuff back to 2 corresponding divs, the first upload button works but the second creates a TypeError and it says…

Traceback (most recent call last):
  File "/comparison.py", line 94, in update_output
    parse_contents(c, n, d ) for c, n, d in zip(list_of_contents, list_of_names, list_of_dates)]
TypeError: 'float' object is not iterable

the code (i mean parse_contents() ) are pretty much identical to the example in the documentation and that bit seems to work fine. It’s the list comprehension bit that is not working. I’m not sure where the float is coming from.

I guess one of them is not a real list.