I have for a number of years been using this pattern when calling a method on jsValue to handle optional parameters: public func doit(_ options: [String : Any]?) { jsValueOutline.invokeMethod("doit", withArguments: [options ?? [:]]) } In particular notice the code: `options ?? [:]` This stated crashing for me recently with this stack: Thread 0 Crashed:: Dispatch queue: com.apple.main-thread 0 libobjc.A.dylib 0x00007fff6eb583c9 class_conformsToProtocol + 170 1 com.apple.JavaScriptCore 0x00007fff3858d6bd objectToValueWithoutCopy(JSContext*, objc_object*) + 77 2 com.apple.JavaScriptCore 0x00007fff3858cf3a objectToValue(JSContext*, objc_object*) + 74 3 com.apple.JavaScriptCore 0x00007fff3858ed17 -[JSValue invokeMethod:withArguments:] + 151 I "think" it's related to 10.15.4 beta, but not entirely sure. It's odd in that I couldn't reproduce it for a day (I was getting user reports of it) ... and then "all of the sudden" I could reproduce it every time. The only change that I remember making is that I enabled Safari debugger Safari > My Computer > Automatically Show Web Inspector for JSContexts .. and also Pause those contexts. I am able to fix the problem by chaining the above code to: publicfunc doit(_ options: [String : Any]?) { jsValueOutline.invokeMethod("doit", withArguments: [options as Any]) } Note that the `options ?? [:]` code is now changed to `options as Any`. I've posted a thread on this issue in the developer forums: https://forums.developer.apple.com/thread/129058