typescript error: Element implicitly has an 'any' type

Question

Typescript newb here. I am getting an error on updatedState[key1].

When I hover over updatedState I get error Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'classTitleObjType'. No index signature with a parameter of type 'string' was found on type 'classTitleObjType'.

and when I hover over key1 I get error Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'classTitleObjType'. No index signature with a parameter of type 'string' was found on type 'classTitleObjType'.ts(7053)

With updatedState[key1] I am trying to assign a value to that key. key1 is a variable. Also updatedState is assigned classTitleObjType as its type. So doesn't updateState know the only thing type that can access it dynamically is the string 'byID' or 'allID'

interface byID {
  [key: string] : {
    id: string,
    classTitle: string
  }
}

interface classTitleObjType {
  byID : byID,
  allID : string[]
}

const classTitleObj: classTitleObjType = {
  byID : {
    "class0": {
      id: "class0",
      classTitle: 'PsychologyRedux'
    },
    "class1": {
      id: "class1",
      classTitle: 'HistoryRedux',
    } 
  },
  allID: ["class0", "class1"]
}

interface ADD_CLASS_ACTION {
  type: 'ADD_CLASS',
  payload: string
}

const classesReducer = (state: classTitleObjType = classTitleObj, action: ADD_CLASS_ACTION) => {
  switch(action.type){
    case 'ADD_CLASS':
      let classTitleToAdd = action.payload

      let updatedState: classTitleObjType = {
        byID : {},
        allID: []
      }

      for(const key1 in state){
        
        if(Array.isArray(state[key1 as keyof classTitleObjType])){
          updatedState[key1] = [...state[key1]]

        } else {

          updatedState[key1] = {...state[key1]}

          for (const key2 in state[key1]){
            updatedState[key1][key2] = {...state[key1][key2]}
          }
        }
      }

      let newNum = Object.keys(state.byID).length
      let newID = "class" + newNum

      updatedState.byID[newID] = {
        id: newID,
        classTitle: classTitleToAdd
      }
  
      updatedState.allID.push(newID)

      return updatedState;

    default:
      return state
  }
}

export default classesReducer

I thought as keyof might be the answer, but no matter what I try ([key1 as keyof classTitleObjType], [key1 as keyof state], [key1 as keyof updatedState] the error never resolves.