I'm having issues with contradiction in my code

Rasa Open Source
1 
- rule: ask about previous improvements default
  steps:
    - intent: ask_about_previous_improvements
    - slot_was_set:
        - update: true
    - action: utter_affirm_response
    - action: utter_previous_improvements


- rule: ask about previous improvements without update
  steps:
    - intent: ask_about_previous_improvements
    - slot_was_set:
        - update: null
    - action: utter_deny_response
    - action: utter_ask_for_improvements