Entity extraction, synonyms

I’m trying to map my entities to one value, to fill my slot.

Below you find a part of my intent

## intent: histo_uit
  - historische data importeren uit [unit4](histo_uit:unit4)
  - data importeren uit [unit 4 audition](histo_uit:unit4)
  - gegevens uploaden vanuit [unit 4](histo_uit:unit4)
  - data importeren uit [UNIT4Audition](histo_uit:unit4)

My nlu config:

- name: "tokenizer_whitespace"
- name: "ner_crf"
- name: "intent_featurizer_count_vectors"
- name: "intent_classifier_tensorflow_embedding"
  intent_tokenization_flag: true
  intent_split_symbol: "+"

However. when I type: “historische data importeren uit unit 4”, the slot histo_uit get’s the value unit 4 (with the whitespace) in stead of unit4.

Can someone tell me why it is not working as I expected?

Found it already, I had to put ner_synonyms in my nlu config.