Minetest error

#0  0x00000000006119e7 in MapgenV6::generateCaves(int) ()
#1  0x0000000000612f5c in MapgenV6::makeChunk(BlockMakeData*) ()
#2  0x0000000000604189 in EmergeThread::Thread() ()
#3  0x0000000000ad51b7 in JThread::TheThread(void*) ()
#4  0x00007ffff6696e9a in start_thread ()
   from /lib/x86_64-linux-gnu/libpthread.so.0
#5  0x00007ffff5343cbd in clone () from /lib/x86_64-linux-gnu/libc.so.6
#6  0x0000000000000000 in ?? ()