Increment default constructed int in the unordered map

I have unordered map of counters, e.g:

std::unordered_map<std::string, std::size_t> counters_;

Do i need to manually create a value before trying to increment it? Will the next line be considered undefined behavior?

std::unordered_map<std::string, std::size_t> counters_;

counters_["non_existing_key"] += 1;