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Variable-length array in TTree if length variable is set to zero?

How does ROOT handle the situation with a variable length array where the dimension has been set to zero? I have a TTree with an old-style variable length array:

Int_t nChan;
Double_t EChan[12];
eventsum->Branch("nChan", &nChan, "nChan/I");
eventsum->Branch("EChan", EChan, "EChan[nChan]/D");

On some events, there may not be any of this channel data, in which case nChan=0. How does ROOT handle that situation on writing? I’m getting a segfault “near” where this data gets filled and written out within each event:

#6  0x00002b0a46432bf0 in TBufferFile::WriteFastArray(double const*, int) () from /sw/eb/sw/ROOT/6.14.06-foss-2018b-Python-3.6.6/lib/libRIO.so
#7  0x00002b0a471efb4c in TBranch::FillLeavesImpl(TBuffer&) () from /sw/eb/sw/ROOT/6.14.06-foss-2018b-Python-3.6.6/lib/libTree.so
#8  0x00002b0a471f56b3 in TBranch::FillImpl(ROOT::Internal::TBranchIMTHelper*) [clone .localalias.114] () from /sw/eb/sw/ROOT/6.14.06-foss-2018b-Python-3.6.6/lib/libTree.so
#9  0x00002b0a472611f8 in TTree::Fill() () from /sw/eb/sw/ROOT/6.14.06-foss-2018b-Python-3.6.6/lib/libTree.so
#10 0x000000000051029f in CDMSSimRoot::SaveEventData (this=0x23119d0) at CDMSSimRoot.cc:561

Unfortunately this is in a multithreaded application (with all the ROOT stuff protected behind efficiency killing mutexes), and happens after an hour or more into the job. I’m trying to infer the cause, and this seemed like one of the possibilities.

I would prefer not to have to convert these data structures to vectors, as we have a large amount of past simulation data already written, and changing the TTree structure will mess up downstream analysis code.

ROOT Version: 6.14
Platform: MacOS, LInux
Compiler: GCC, LLVM

Hi @Michael_Kelsey,
I think @pcanal can give you details about that.