No index signature with a parameter of type 'string' was found on type

Получаем ошибку:

Element implicitly has an 'any' type because expression of type
 'string' can't be used to index type 'RootActionTypesListType'.
  No index signature with a parameter of type 'string' 
was found on type 'RootActionTypesListType'.  TS7053

    11 | 
    12 | for (const key of Object.keys(RootActionTypes)) {
  > 13 |     RootActionTypes[key] = 'ROOT_' + key;
       |     ^
    14 | }
    15 | RootActionTypes = Object.freeze(RootActionTypes);
    16 |

Для кода вида: