Uninitialized scalar field

The field will contain an arbitrary value left over from earlier computations. A scalar field is not initialized by the constructor
/src/mongo/db/s/session_catalog_migration_source.h:311: UNINIT_CTOR 134243 Class member declaration for "_averageSessionDocSize".
/src/mongo/db/s/session_catalog_migration_source.cpp:216: UNINIT_CTOR 134243 Non-static class member "_averageSessionDocSize" is not initialized in this constructor nor in any functions that it calls.