fast-bert: learner.save_model gives KeyError while saving tokenizer/vocab file

I’m trying to run the multilabel classification model and while saving the model it give me an error on vocab file learner.save_model() gives below error:

Is this because I have not specified some path or because I’m not using a pretrained model path from local as in sample notebook.

My learner config is as below:

DataBunchConfig as below:

Any help appreciated. Thanks!