hadd crashes when recompressing certain input files

Some combinations of my small root files (which contain a TTree and some histograms) cannot be merged by hadd if compression options are given (-f6). If I remove this option the files can be merged.

MWE:

hadd -k -f6 test.root /eos/lhcb/user/o/olupton/ROOT/180507/input{1,2}.root

(should be world readable)

hadd Target file: test.root
hadd compression setting for all ouput: 6
hadd Source file 1: /eos/lhcb/user/o/olupton/ROOT/180507/input1.root
hadd Source file 2: /eos/lhcb/user/o/olupton/ROOT/180507/input2.root
hadd Sources and Target have different compression levels
hadd merging will be slower
hadd Target path: test.root:/
hadd Target path: test.root:/ccbar*** Error in `hadd': free(): invalid next size (fast): 0x0000000001ca7540 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x7c619)[0x7f56b5aac619]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN6TLeafFD1Ev+0x5a)[0x7f56b7fdc428]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN6TLeafFD0Ev+0x18)[0x7f56b7fdc450]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libCore.so(_ZN11TCollection14GarbageCollectEP7TObject+0x14b)[0x7f56b6c61831]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libCore.so(_ZN9TObjArray6DeleteEPKc+0xfd)[0x7f56b6c72011]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN7TBranchD2Ev+0x225)[0x7f56b7f81c83]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN7TBranchD0Ev+0x18)[0x7f56b7f81eba]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libCore.so(_ZN11TCollection14GarbageCollectEP7TObject+0x14b)[0x7f56b6c61831]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libCore.so(_ZN9TObjArray6DeleteEPKc+0xfd)[0x7f56b6c72011]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN5TTreeD1Ev+0x1ff)[0x7f56b7fe94ef]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(_ZN5TTreeD0Ev+0x18)[0x7f56b7fe9a3a]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libTree.so(+0xb626e)[0x7f56b7f6c26e]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libCore.so(_ZN6TClass10DestructorEPvb+0xaf)[0x7f56b6cb4c19]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libRIO.so(_ZN11TFileMerger14MergeRecursiveEP10TDirectoryP5TListi+0x2156)[0x7f56b773a1d0]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libRIO.so(_ZN11TFileMerger14MergeRecursiveEP10TDirectoryP5TListi+0xdc3)[0x7f56b7738e3d]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libRIO.so(_ZN11TFileMerger12PartialMergeEi+0x634)[0x7f56b773aa88]
/cvmfs/sft.cern.ch/lcg/views/LCG_93/x86_64-centos7-gcc62-dbg/lib/libRIO.so(_ZN11TFileMerger5MergeEb+0x2f)[0x7f56b7738077]
hadd[0x40b996]
hadd[0x40bd58]
hadd[0x40e459]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7f56b5a51c05]
hadd[0x40b759]
======= Memory map: ========
00400000-0041c000 r-xp 00000000 00:34 81325692                           /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/bin/hadd
0061c000-0061d000 r--p 0001c000 00:34 81325692                           /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/bin/hadd
0061d000-0061e000 rw-p 0001d000 00:34 81325692                           /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/bin/hadd
01b31000-0777e000 rw-p 00000000 00:00 0                                  [heap]
7f56a0000000-7f56a0021000 rw-p 00000000 00:00 0
7f56a0021000-7f56a4000000 ---p 00000000 00:00 0
7f56a7559000-7f56a78db000 rw-p 00000000 00:00 0
7f56a78db000-7f56a7c0f000 r-xp 00000000 00:34 81327936                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libTreePlayer.so
7f56a7c0f000-7f56a7e0f000 ---p 00334000 00:34 81327936                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libTreePlayer.so
7f56a7e0f000-7f56a7e1a000 r--p 00334000 00:34 81327936                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libTreePlayer.so
7f56a7e1a000-7f56a7e29000 rw-p 0033f000 00:34 81327936                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libTreePlayer.so
7f56a7e29000-7f56a7e31000 rw-p 00000000 00:00 0
7f56a7e31000-7f56a7e52000 r-xp 00000000 fd:01 2098511                    /usr/lib64/ld-2.17.so
7f56a7e52000-7f56a8052000 ---p 00021000 fd:01 2098511                    /usr/lib64/ld-2.17.so
7f56a8052000-7f56a8053000 r--p 00021000 fd:01 2098511                    /usr/lib64/ld-2.17.so
7f56a8053000-7f56a8054000 rw-p 00022000 fd:01 2098511                    /usr/lib64/ld-2.17.so
7f56a8054000-7f56a8d65000 rw-p 00000000 00:00 0
7f56a8d65000-7f56aedde000 r--p 00000000 00:34 81326688                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/etc/allDict.cxx.pch
7f56aedde000-7f56aee03000 r-xp 00000000 fd:01 2098592                    /usr/lib64/libtinfo.so.5.9
7f56aee03000-7f56af003000 ---p 00025000 fd:01 2098592                    /usr/lib64/libtinfo.so.5.9
7f56af003000-7f56af007000 r--p 00025000 fd:01 2098592                    /usr/lib64/libtinfo.so.5.9
7f56af007000-7f56af008000 rw-p 00029000 fd:01 2098592                    /usr/lib64/libtinfo.so.5.9
7f56af008000-7f56b1deb000 r-xp 00000000 00:34 81328177                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libCling.so
7f56b1deb000-7f56b1feb000 ---p 02de3000 00:34 81328177                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libCling.so
7f56b1feb000-7f56b21cd000 r--p 02de3000 00:34 81328177                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libCling.so
7f56b21cd000-7f56b21dc000 rw-p 02fc5000 00:34 81328177                   /cvmfs/sft.cern.ch/lcg/releases/ROOT/6.12.06-51921/x86_64-centos7-gcc62-dbg/lib/libCling.so
7f56b21dc000-7f56b2206000 rw-p 00000000 00:00 0
7f56b2206000-7f56b2a0b000 r--s 00000000 fd:01 23073069                   /var/lib/sss/mc/passwd
7f56b2a0b000-7f56b2a13000 r-xp 00000000 fd:01 2350610                    /usr/lib64/libnss_sss.so.2
7f56b2a13000-7f56b2c12000 ---p 00008000 fd:01 2350610                    /usr/lib64/libnss_sss.so.2
7f56b2c12000-7f56b2c13000 r--p 00007000 fd:01 2350610                    /usr/lib64/libnss_sss.so.2
7f56b2c13000-7f56b2c14000 rw-p 00008000 fd:01 2350610                    /usr/lib64/libnss_sss.so.2
7f56b2c14000-7f56b2c20000 r-xp 00000000 fd:01 2098536                    /usr/lib64/libnss_files-2.17.so
7f56b2c20000-7f56b2e1f000 ---p 0000c000 fd:01 2098536                    /usr/lib64/libnss_files-2.17.so
7f56b2e1f000-7f56b2e20000 r--p 0000b000 fd:01 2098536                    /usr/lib64/libnss_files-2.17.so
7f56b2e20000-7f56b2e21000 rw-p 0000c000 fd:01 2098536                    /usr/lib64/libnss_files-2.17.so