Unable to retrieve package versions for 'System.Slices'

Trying to install the Spreads 0.5.1 in Visual Studio Code on Ubuntu 16.04, my paket.dependency file is:

source https://www.nuget.org/api/v2/

framework: >= net461
nuget FSharp.Core

running paket add Spreads --version 0.6.1 I get:

Paket version 5.132.4
Adding Spreads 0.5.1 to /home/tom/F#/paket.dependencies into group Main
Resolving packages for group Main:
 - Spreads 0.5.1
 - FSharp.Core 4.2.3
 - Spreads.Core 0.6.1
 - Spreads.Extensions 0.5.1
Trial1 (NuGet.GetVersions) did not yield any results, trying again.
   https://www.nuget.org/api/v2/FindPackagesById()?semVerLevel=2.0.0&id='System.Slices'
   https://www.nuget.org/api/v2/Packages?semVerLevel=2.0.0&$filter=Id eq 'System.Slices'
 - Newtonsoft.Json 10.0.3
 - System.Memory 4.4.0-preview2-25405-01
     System.Memory 4.4.0-preview2-25405-01 was unlisted
 - System.Memory 4.4.0-preview1-25305-02
 - System.Runtime.CompilerServices.Unsafe 4.4.0
 - System.Numerics.Vectors 4.4.0
 - System.Buffers 4.4.0
 - System.Threading.Tasks.Extensions 4.4.0
 - NodaTime 2.2.3
Performance:
 - Resolver: 4 seconds (1 runs)
    - Runtime: 181 milliseconds
    - Blocked (retrieving package details): 506 milliseconds (6 times)
    - Blocked (retrieving package versions): 3 seconds (5 times)
    - Not Blocked (retrieving package versions): 6 times
    - Not Blocked (retrieving package details): 6 times
 - Average Request Time: 171 milliseconds
 - Number of Requests: 23
 - Runtime: 4 seconds
Paket failed with
-> Unable to retrieve package versions for 'System.Slices'
   -- CLOSED --
      Spreads 0.5.1 (from /home/tom/F#/paket.dependencies)
      FSharp.Core  (from /home/tom/F#/paket.dependencies)
      Spreads.Core >= 0.5.1 (from Spreads 0.5.1)
      Spreads.Extensions >= 0.5.1 (from Spreads 0.5.1)
      Newtonsoft.Json >= 9.0.1 (from Spreads.Core 0.6.1)
      Newtonsoft.Json >= 7.0.1 (from Spreads.Extensions 0.5.1)
      System.Memory >= 4.4.0-beta-24911-06 (from Spreads.Core 0.6.1)
      System.Runtime.CompilerServices.Unsafe >= 4.4.0-beta-24911-06 (from Spreads.Core 0.6.1)
      System.Numerics.Vectors >= 4.1.1 (from Spreads.Core 0.6.1)
      System.Buffers >= 4.0 (from Spreads.Core 0.6.1)
      System.Threading.Tasks.Extensions >= 4.0 (from Spreads.Core 0.6.1)
      NodaTime >= 1.3.1 (from Spreads.Extensions 0.5.1)
   -- OPEN ----
      System.Slices >= 0.1.0-e170111-4 (from Spreads.Core 0.6.1)
      System.Numerics.Vectors >= 4.4.0-preview1-25305-02 (from System.Memory 4.4.0-preview1-25305-02)
      System.Runtime.CompilerServices.Unsafe >= 4.4.0-preview1-25305-02 (from System.Memory 4.4.0-preview1-25305-02)
      System.Diagnostics.Debug >= 4.3 (from System.Buffers 4.4.0)
      System.Diagnostics.Tracing >= 4.3 (from System.Buffers 4.4.0)
      System.Resources.ResourceManager >= 4.3 (from System.Buffers 4.4.0)
      System.Runtime >= 4.3 (from System.Buffers 4.4.0)
      System.Threading >= 4.3 (from System.Buffers 4.4.0)
      System.Collections >= 4.3 (from System.Threading.Tasks.Extensions 4.4.0)
      System.Runtime >= 4.3 (from System.Threading.Tasks.Extensions 4.4.0)
      System.Threading.Tasks >= 4.3 (from System.Threading.Tasks.Extensions 4.4.0)
      NETStandard.Library >= 1.6.1 (from System.Memory 4.4.0-preview1-25305-02)
      NETStandard.Library >= 1.6.1 (from System.Runtime.CompilerServices.Unsafe 4.4.0)
-> Could not find versions for package System.Slices on https://www.nuget.org/api/v2.
-> Source 'https://www.nuget.org/api/v2' yielded (0): []
    -  Request 'https://www.nuget.org/api/v2/FindPackagesById()?semVerLevel=2.0.0&id='System.Slices'' finished with: []
    -  Request 'https://www.nuget.org/api/v2/Packages?semVerLevel=2.0.0&$filter=Id eq 'System.Slices'' was cancelled (another one was faster)

Is there something I'm doing wrong?

ionide / ionide-vscode-paket

Unable to retrieve package versions for 'System.Slices' #50