If WKContextPostMessageToInjectedBundle is called after the web process has exited, it will crash. Here's the backtrace: WebKit.dll!CoreIPC::Connection::isValid() Line 122 + 0x11 bytes C++ WebKit.dll!CoreIPC::Connection::sendMessage(CoreIPC::MessageID messageID={...}, WTF::PassOwnPtr<CoreIPC::ArgumentEncoder> arguments={...}) Line 82 + 0x8 bytes C++ WebKit.dll!WebKit::WebProcessProxy::sendMessage(CoreIPC::MessageID messageID={...}, WTF::PassOwnPtr<CoreIPC::ArgumentEncoder> arguments={...}) Line 125 + 0x22 bytes C++ WebKit.dll!WebKit::WebProcessProxy::send<enum WebProcessMessage::Kind,CoreIPC::Arguments1<WebCore::String const &> >(WebProcessMessage::Kind messageID=PostMessageW, unsigned __int64 destinationID=0, const CoreIPC::Arguments1<WebCore::String const &> & arguments={...}) Line 138 + 0x23 bytes C++ WebKit.dll!WebKit::WebContext::postMessageToInjectedBundle(WebCore::StringImpl * message=0x045a6db8) Line 165 + 0x32 bytes C++ > WebKit.dll!WKContextPostMessageToInjectedBundle(OpaqueWKContext * contextRef=0x045a1890, OpaqueWKStringRef * messageRef=0x045a6db8) Line 79 C++ WebKitTestRunner.exe!WTR::TestInvocation::invoke() Line 112 + 0x1e bytes C++ WebKitTestRunner.exe!WTR::TestController::runTest(const char * test=0x045a11f0) Line 131 C++ WebKitTestRunner.exe!WTR::TestController::run() Line 155 + 0x29 bytes C++ WebKitTestRunner.exe!main(int argc=4, const char * * argv=0x045a0eb8) Line 33 C++ WebKitTestRunner.exe!__tmainCRTStartup() Line 597 + 0x17 bytes C kernel32.dll!_BaseProcessStart@4() + 0x23 bytes WebContext::m_process is non-null, but m_process->isValid() is false. WebContext::postMessageToInjectedBundle doesn't check this before calling into m_process, though, leading to the crash.