Could not load signature of WebSocket


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Could not load signature of WebSocket
Unloading broken assembly Assets/Plugins/Photon3DotNet.dll, this assembly can cause crashes in the runtime
Unable to resolve reference 'websocket-sharp'. Is the assembly missing or incompatible with the current platform?
Reference validation can be disabled in the Plugin Inspector.
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Hi, I got some problem when I add Photon3DotNet.dll to Plugins folder

dll version: Photon-OnPremise-Server-SDK_v4-0-29-11263
Unity version: Unity 2019.4.20f1c1 Personal
Unity Api Compatibility Level: .NET Standard 2.0(I try to use .NET 4.x, same problem)

Thanks!