Found via UBSan, actual errors (addresses not pruned for illustrative purposes):
"runtime error: store to misaligned address 0x7f9a9178e84c for type
'size_t' (aka 'unsigned long'), which requires 8 byte alignment"
"runtime error: load of misaligned address 0x7f9a9140f2cf for type
'size_t' (aka 'unsigned long'), which requires 8 byte alignment"
c680b3e115