Why do I get “Reducer […] returned undefined during initialization” despite providing initialState to createStore()?

I had the same error but I didn’t include a default case

function generate(state={} ,action) {
  switch (action.type) {
    case randomNumber:
      return {
        ...state,
        random: action.payload
      }   
    default: // need this for default case
      return state 
   }
}