OBSERVE crashes during rebalance

Implementation is based on yours:

https://github.com/pavel-paulau/cbagent/blob/master/cbagent/collectors/libstats/pool.py

During rebalance I got:
python-couchbase: self->nremaining at src/callbacks.c:106. AbortAborted (core dumped)

(gdb) bt
#0 0x00007f2ada467f77 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1 0x00007f2ada46b5e8 in __GI_abort () at abort.c:90
#2 0x00007f2ad6ec61e0 in pycbc_handle_assert (msg=msg@entry=0x7f2ad6ed3b04 "self->nremaining", file=file@entry=0x7f2ad6ed3aa0 "src/callbacks.c", line=line@entry=106)
at src/exceptions.c:25
#3 0x00007f2ad6ec7d38 in maybe_breakout (self=<optimized out>, self=<optimized out>) at src/callbacks.c:106
#4 0x00007f2ad6ec8529 in observe_callback (instance=<optimized out>, cookie=<optimized out>, err=<optimized out>, resp=0x7f2ab3ffd8d0) at src/callbacks.c:569
#5 0x00007f2ad6c9abc1 in observe_response_handler (server=0x7f2aa805efc0, command_data=<optimized out>, res=<optimized out>) at src/handler.c:600
#6 0x00007f2ad6c9b398 in lcb_dispatch_response (c=0x7f2aa805efc0, ct=0x7f2ab3ffdd40, header=0x7f2aa804f000) at src/handler.c:1268
#7 0x00007f2ad6ca32e5 in lcb_proto_parse_single (c=0x7f2aa805efc0, stop=1389655096344192000) at src/server_parse.c:215
#8 0x00007f2ad6ca2def in do_read_data (allow_read=<optimized out>, c=<optimized out>) at src/server_io.c:45
#9 lcb_server_v0_event_handler (sock=<optimized out>, which=<optimized out>, arg=0x7f2aa805efc0) at src/server_io.c:107
#10 0x00007f2ad6c941c5 in lcb_io_run_event_loop (iops=<optimized out>) at plugins/io/select/plugin-select.c:562
#11 0x00007f2ad6ca592d in lcb_wait (instance=0x26fc130) at src/wait.c:77
#12 0x00007f2ad6ecdeb3 in pycbc_oputil_wait_common (self=self@entry=0x225f050) at src/oputil.c:510
#13 0x00007f2ad6ecdf30 in pycbc_common_vars_wait (cv=cv@entry=0x7f2ab3ffe000, self=self@entry=0x225f050) at src/oputil.c:59
#14 0x00007f2ad6ed05c3 in observe_common (self=0x225f050, args=<optimized out>, kwargs=<optimized out>, argopts=<optimized out>) at src/observe.c:214
#15 0x000000000053bf0b in PyEval_CallObjectWithKeywords ()
#16 0x0000000000595e0c in ?? ()
#17 0x000000000052e3cb in PyEval_EvalFrameEx ()
#18 0x000000000052e672 in PyEval_EvalFrameEx ()
#19 0x000000000052e672 in PyEval_EvalFrameEx ()
#20 0x000000000050697a in ?? ()
#21 0x00000000005301ae in PyEval_EvalFrameEx ()
#22 0x000000000052e672 in PyEval_EvalFrameEx ()
#23 0x000000000052e672 in PyEval_EvalFrameEx ()
#24 0x000000000050697a in ?? ()
#25 0x00000000004d53f4 in ?? ()
#26 0x000000000053bf0b in PyEval_CallObjectWithKeywords ()
#27 0x0000000000579a22 in _start ()