unordered_map emplace with default constructor

I have unordered_map with key value pairs of type string and aClass respectively; where aClass can not be moved(it has a mutex). I don't want it to be copy-constructed either; I think it wouldn't be wise to copy-construct a class that contains a mutex. To delay the construction of the item until its insertion to the map, I have tried to insert it to the map using emplace, therefore, the second argument must be empty:

aList.emplace("aString");

however, the previous line did not work. Any ideas how to emplace using the default constructor? I have also tried:

aList.emplace("aString", void);
aList.emplace(std::piecewise_construct,"aString");

Thank you,