MongoDB 2.8 RC4 + wiredtiger fails to sync with 2.6 replicaset

We have a big replicaset ( 300G data + 200G index) and want to benefit from the wiredtiger compression. We add a MongoDB 2.8 RC4 node into the existing replicaset. From the log, it successfully clone most data and fails to create index for a mapreduce temp collection, full log is in the attachments. There are no problem for us to add a new 2.6.6 node. Is this a bug in 2.8 RC4?

2014-12-31T23:06:43.444+0000 I INDEX    [repl writer worker 5] build index on: data.tmp.mr.raw.traffic.usage_222206 properties: { v: 1, key: { value.timestamp: 1 }, name: "value.timestamp_1", ns: "data.tmp.mr.raw.traffic.usage_222206" }

2014-12-31T23:06:43.444+0000 I INDEX    [repl writer worker 5]  building index using bulk method

2014-12-31T23:06:43.447+0000 I INDEX    [repl writer worker 5] build index done.  scanned 0 total records. 0 secs

2014-12-31T23:06:43.447+0000 E INDEX    [repl index builder 45] IndexBuilder could not build index: WriteConflict WriteConflict

2014-12-31T23:06:43.447+0000 I -        [repl index builder 45] Fatal Assertion 28555

2014-12-31T23:06:43.453+0000 I CONTROL  [repl index builder 45] 

 0xf133b9 0xebd1d1 0xea2a01 0xa84d9d 0xea5680 0xf5f6c4 0x7f05d6384182 0x7f05d548530d

----- BEGIN BACKTRACE -----

{"backtrace":[{"b":"400000","o":"B133B9"},{"b":"400000","o":"ABD1D1"},{"b":"400000","o":"AA2A01"},{"b":"400000","o":"684D9D"},{"b":"400000","o":"AA5680"},{"b":"400000","o":"B5F6C4"},{"b":"7F05D637C000","o":"8182"},{"b":"7F05D538A000","o":"FB30D"}],"processInfo":{ "mongodbVersion" : "2.8.0-rc4", "gitVersion" : "3ad571742911f04b307f0071979425511c4f2570", "uname" : { "sysname" : "Linux", "release" : "3.13.0-29-generic", "version" : "#53-Ubuntu SMP Wed Jun 4 21:00:20 UTC 2014", "machine" : "x86_64" }, "somap" : [ { "elfType" : 2, "b" : "400000" }, { "b" : "7FFF85EFE000", "elfType" : 3 }, { "b" : "7F05D637C000", "path" : "/lib/x86_64-linux-gnu/libpthread.so.0", "elfType" : 3 }, { "b" : "7F05D6174000", "path" : "/lib/x86_64-linux-gnu/librt.so.1", "elfType" : 3 }, { "b" : "7F05D5F70000", "path" : "/lib/x86_64-linux-gnu/libdl.so.2", "elfType" : 3 }, { "b" : "7F05D5C6C000", "path" : "/usr/lib/x86_64-linux-gnu/libstdc++.so.6", "elfType" : 3 }, { "b" : "7F05D5966000", "path" : "/lib/x86_64-linux-gnu/libm.so.6", "elfType" : 3 }, { "b" : "7F05D5750000", "path" : "/lib/x86_64-linux-gnu/libgcc_s.so.1", "elfType" : 3 }, { "b" : "7F05D538A000", "path" : "/lib/x86_64-linux-gnu/libc.so.6", "elfType" : 3 }, { "b" : "7F05D659A000", "path" : "/lib64/ld-linux-x86-64.so.2", "elfType" : 3 } ] }}

 mongod(_ZN5mongo15printStackTraceERSo+0x29) [0xf133b9]

 mongod(_ZN5mongo10logContextEPKc+0xE1) [0xebd1d1]

 mongod(_ZN5mongo13fassertFailedEi+0x61) [0xea2a01]

 mongod(_ZN5mongo12IndexBuilder3runEv+0x56D) [0xa84d9d]

 mongod(_ZN5mongo13BackgroundJob7jobBodyEv+0x120) [0xea5680]

 mongod(+0xB5F6C4) [0xf5f6c4]

 libpthread.so.0(+0x8182) [0x7f05d6384182]

 libc.so.6(clone+0x6D) [0x7f05d548530d]

-----  END BACKTRACE  -----

2014-12-31T23:06:43.453+0000 I -        [repl index builder 45] 

***aborting after fassert() failure

mongod.data.2.8.log.2015-01-01T01-45-36