Discretization error

Hi, the following code

```
num_durations = 10
labtrans = MTLR.label_transform(num_durations)
get_target = lambda df: (df['time'].values, df['event'].values)
y_train = labtrans.fit_transform(*get_target(data_train))
y_val = labtrans.transform(*get_target(data_val))
y_test = labtrans.transform(*get_target(data_test))
```

gives me the error:

`TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'`

It is curious that the same code works smoothly before but it no longer works now that I have changed the dataframe features (the columns `time` and `event` have remained the same, without null values, ​​and therefore the problem should not arise).
A suggestion as to what the error might be in such cases would be ideal because I cannot understand what causes it.

Thanks in advance and congratulations on your excellent work.

EDIT: the column `event` had a negative value, which gave rise to the error.

Start at MTLR.label_transform and the fit_transform call shown in the report. Reproduce the error with the supplied time and event arrays, then inspect the event values; the report's edit identifies a negative event as the cause. Done means confirming the invalid input as the source of the TypeError.

Written by the indexing model from the issue text.