This is due to the series df[cat]
containing elements that have varying data types e.g.(strings and/or floats). This could be due to the way the data is read, i.e. numbers are read as float and text as strings or the datatype was float and changed after the fillna
operation.
In other words
pandas data type 'Object' indicates mixed types rather than str type
so using the following line:
df[cat] = le.fit_transform(df[cat].astype(str))
should help