Cannot resolve symbol 'nng'

IDE: JetBrains Rider
installation: via NuGet; Subor.nng.NETCore 1.1.1.1

the installed version is old but the project importing seems fine:
```c#

```

however `using nng` not work with message: Cannot resolve symbol 'nng'

the project now is set with target framework: .NETFramework v4.6.1
when tried to add reference, error occured:
>Rider – Can not add reference
Can not read assembly metadata from '...\nng.dll'.

i used to use NNanomsg, which is flawless (install via nuget, and then just coding with `using NNanomsg` and everything works as expected).
i'd like to try this one, but tumbled at the first step, what did i miss?