TypeError:'NoneType' object is not subscriptable while calling topic_model.hierarchical_topics

Hi,
I am trying zero-shot topic modelling with BERTopic. The following fit_transform ran successfully:

    topic_model = BERTopic(
        embedding_model="thenlper/gte-small", 
        min_topic_size=50,
        zeroshot_topic_list=zeroshot_topic_list,
        zeroshot_min_similarity=.85,
        representation_model=KeyBERTInspired()
    )
    topics, _ = topic_model.fit_transform(docs)

topic_model.get_topic_info()

While running hierarchical_topics = topic_model.hierarchical_topics(docs), getting the following error:

TypeError:'NoneType' object is not subscriptable
Open Traceback
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[87], line 1
----> 1 hierarchical_topics = topic_model.hierarchical_topics(docs)
File /mnt/xarfuse/uid-564347/e4f2f620-seed-nspid4026531836_cgpid15010019-ns-4026531841/bertopic/_bertopic.py:975, in BERTopic.hierarchical_topics(self, docs, linkage_function, distance_function)
    972     linkage_function = lambda x: sch.linkage(x, 'ward', optimal_ordering=True)
    974 # Calculate distance
--> 975 embeddings = self.c_tf_idf_[self._outliers:]
    976 X = distance_function(embeddings)
    977 X = validate_distance_matrix(X, embeddings.shape[0])
TypeError: 'NoneType' object is not subscriptable

zeroshot_topic_list contains 251 topics, docs are 15k. Similar code without zeroshot works fine. Please let me know if you have any insights, thanks!

Start in bertopic.py at BERTopic.hierarchical_topics, especially the c_tf_idf access around line 975, and reproduce the failure with the zero-shot configuration shown. Compare it with the non-zero-shot case that works. Done means hierarchical_topics completes for the zero-shot model and returns the expected hierarchy without the NoneType error.