commands used:
./mongod --enableEncryption --encryptionKeyFile key
db.test.insert({a:1})
./mongod # crash

LLDB log:
➜  dev.mongo git:(SERVER-19116) ✗ lldb -- ./mongod
(lldb) target create "./mongod"
Current executable set to './mongod' (x86_64).
(lldb) r
Process 2537 launched: './mongod' (x86_64)
2015-07-06T18:24:55.737-0400 I -        [initandlisten] Detected data files in /data/db created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
2015-07-06T18:24:55.737-0400 I STORAGE  [initandlisten] wiredtiger_open config: create,cache_size=9G,session_max=20000,eviction=(threads_max=4),statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),checkpoint=(wait=60,log_size=2GB),statistics_log=(wait=0),
Process 2537 stopped
* thread #1: tid = 0x7b1b9, 0x000000010077dbbb mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long(this=0x646e616d6d6f63ac) const + 28 at string:1716, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x000000010077dbbb mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long(this=0x646e616d6d6f63ac) const + 28 at string:1716
   1713	
   1714	    _LIBCPP_INLINE_VISIBILITY
   1715	    bool __is_long() const _NOEXCEPT
-> 1716	        {return bool(__r_.first().__s.__size_ & __short_mask);}
   1717	
   1718	#if _LIBCPP_DEBUG_LEVEL >= 2
   1719	
(lldb) bt
* thread #1: tid = 0x7b1b9, 0x000000010077dbbb mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long(this=0x646e616d6d6f63ac) const + 28 at string:1716, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
  * frame #0: 0x000000010077dbbb mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long(this=0x646e616d6d6f63ac) const + 28 at string:1716
    frame #1: 0x000000010077db9f mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::size(this=0x646e616d6d6f63ac) const + 14 at string:1483
    frame #2: 0x000000010077db91 mongod`mongo::EncryptionKeyManager::_acquireSystemKey() [inlined] std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::empty(this=0x646e616d6d6f63ac) const + 14 at string:1497
    frame #3: 0x000000010077db83 mongod`mongo::EncryptionKeyManager::_acquireSystemKey(this=0x0000000104d005d0) + 67 at encryption_key_manager.cpp:192
    frame #4: 0x000000010077d6af mongod`mongo::EncryptionKeyManager::getKey(this=0x0000000104d005d0, keyID=0x00007fff5fbfc5d0) + 95 at encryption_key_manager.cpp:78
    frame #5: 0x00000001007a6bb4 mongod`mongo::(anonymous namespace)::customize(encryptor=0x0000000104d05290, session=0x0000000106800000, encryptConfig=0x00007fff5fbfcb80, customEncryptor=0x00007fff5fbfc938) + 788 at wiredtiger_encryption_callbacks.cpp:92
    frame #6: 0x0000000101b9a019 mongod`__wt_encryptor_config(session=0x0000000106800000, cval=0x00007fff5fbfcaa8, keyid=0x00007fff5fbfca88, cfg_arg=0x00007fff5fbfcb80, kencryptorp=0x00000001058345c0) + 665 at conn_api.c:443
    frame #7: 0x0000000101b9bcab mongod`wiredtiger_open(home=0x000000010218e229, event_handler=0x0000000104d041f0, config=0x0000000104d045a0, wt_connp=0x0000000104d041e8) + 3963 at conn_api.c:2006
    frame #8: 0x0000000100ef9e01 mongod`mongo::WiredTigerKVEngine::WiredTigerKVEngine(this=0x0000000104d041e0, path=0x000000010218e228, extraOpenOptions=0x000000010218e110, durable=true, repair=false) + 5201 at wiredtiger_kv_engine.cpp:121
    frame #9: 0x0000000100efaeac mongod`mongo::WiredTigerKVEngine::WiredTigerKVEngine(this=0x0000000104d041e0, path=0x000000010218e228, extraOpenOptions=0x000000010218e110, durable=true, repair=false) + 76 at wiredtiger_kv_engine.cpp:143
    frame #10: 0x0000000100ef7f77 mongod`mongo::(anonymous namespace)::WiredTigerFactory::create(this=0x0000000103e35350, params=0x000000010218e208, lockFile=0x0000000104d04190) const + 215 at wiredtiger_init.cpp:64
    frame #11: 0x0000000100d72d90 mongod`mongo::ServiceContextMongoD::initializeGlobalStorageEngine(this=0x0000000103e5e000) + 3936 at service_context_d.cpp:144
    frame #12: 0x0000000100001d0d mongod`mongo::_initAndListen(listenPort=27017) + 1021 at db.cpp:416
    frame #13: 0x00000001000014c3 mongod`mongo::initAndListen(listenPort=27017) + 19 at db.cpp:594
    frame #14: 0x0000000100004c03 mongod`mongoDbMain(argc=1, argv=0x00007fff5fbffac8, envp=0x00007fff5fbffad8) + 803 at db.cpp:823
    frame #15: 0x00000001000048ca mongod`main(argc=1, argv=0x00007fff5fbffac8, envp=0x00007fff5fbffad8) + 42 at db.cpp:639
    frame #16: 0x00007fff9c1615c9 libdyld.dylib`start + 1
    frame #17: 0x00007fff9c1615c9 libdyld.dylib`start + 1
(lldb)