ASSERTION FAILED: m_client.mediaType() >= None && m_client.mediaType() <= WebAudio /Volumes/Data/slave/sierra-debug/build/Source/WebCore/platform/audio/PlatformMediaSession.cpp(84) : WebCore::PlatformMediaSession::PlatformMediaSession(WebCore::PlatformMediaSessionClient &) 1 0x11d068f4d WTFCrash 2 0x1132b818e WebCore::PlatformMediaSession::PlatformMediaSession(WebCore::PlatformMediaSessionClient&) 3 0x1132b830d WebCore::PlatformMediaSession::PlatformMediaSession(WebCore::PlatformMediaSessionClient&) 4 0x1132b8002 WebCore::PlatformMediaSession::create(WebCore::PlatformMediaSessionClient&) 5 0x11309f8ac WebCore::MediaStream::MediaStream(WebCore::ScriptExecutionContext&, WTF::Vector<WTF::RefPtr<WebCore::MediaStreamTrack>, 0ul, WTF::CrashOnOverflow, 16ul> const&) 6 0x11309f5a5 WebCore::MediaStream::MediaStream(WebCore::ScriptExecutionContext&, WTF::Vector<WTF::RefPtr<WebCore::MediaStreamTrack>, 0ul, WTF::CrashOnOverflow, 16ul> const&) 7 0x11309f5ef WebCore::MediaStream::create(WebCore::ScriptExecutionContext&, WTF::Vector<WTF::RefPtr<WebCore::MediaStreamTrack>, 0ul, WTF::CrashOnOverflow, 16ul> const&) 8 0x1130a0db6 WebCore::MediaStream::clone() 9 0x1128b1b46 WebCore::jsMediaStreamPrototypeFunctionCloneCaller(JSC::ExecState*, WebCore::JSMediaStream*, JSC::ThrowScope&) 10 0x1128ae1a2 long long WebCore::BindingCaller<WebCore::JSMediaStream>::callOperation<&(WebCore::jsMediaStreamPrototypeFunctionCloneCaller(JSC::ExecState*, WebCore::JSMediaStream*, JSC::ThrowScope&)), (WebCore::CastedThisErrorBehavior)0>(JSC::ExecState*, char const*) 11 0x1128adefc WebCore::jsMediaStreamPrototypeFunctionClone(JSC::ExecState*) 12 0x2aaba6201028 13 0x11cbe31af llint_entry 14 0x11cbdba7e vmEntryToJavaScript 15 0x11c9b715e JSC::JITCode::execute(JSC::VM*, JSC::ProtoCallFrame*) 16 0x11c9660ee JSC::Interpreter::execute(JSC::EvalExecutable*, JSC::ExecState*, JSC::JSValue, JSC::JSScope*) 17 0x11c964e31 JSC::eval(JSC::ExecState*) 18 0x11cbd5aa3 llint_slow_path_call_eval 19 0x11cbe3a40 llint_entry 20 0x11cbe31af llint_entry 21 0x11cbe31af llint_entry 22 0x11cbdba7e vmEntryToJavaScript 23 0x11c9b715e JSC::JITCode::execute(JSC::VM*, JSC::ProtoCallFrame*) 24 0x11c96a582 JSC::Interpreter::executeCall(JSC::ExecState*, JSC::JSObject*, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) 25 0x11c142ff8 JSC::call(JSC::ExecState*, JSC::JSValue, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) 26 0x11c1432aa JSC::profiledCall(JSC::ExecState*, JSC::ProfilingReason, JSC::JSValue, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) 27 0x11caa40f8 JSC::JSJobMicrotask::run(JSC::ExecState*) 28 0x11249e2e7 WebCore::JSMainThreadExecState::runTask(JSC::ExecState*, JSC::Microtask&) 29 0x1125ced74 WebCore::JSDOMWindowMicrotaskCallback::call() 30 0x1125cec5d WebCore::JSDOMWindowBase::queueTaskToEventLoop(JSC::JSGlobalObject const*, WTF::Ref<JSC::Microtask>&&)::$_0::operator()() 31 0x1125ceb09 WTF::Function<void ()>::CallableWrapper<WebCore::JSDOMWindowBase::queueTaskToEventLoop(JSC::JSGlobalObject const*, WTF::Ref<JSC::Microtask>&&)::$_0>::call() https://build.webkit.org/results/Apple%20Sierra%20Debug%20WK2%20(Tests)/r215273%20(444)/results.html
<rdar://problem/31586113>
I fixed this in r215300: <http://trac.webkit.org/changeset/215300>