Got a "not a valid VSIX package" error:

1/25/2018 7:29:06 PM - Microsoft VSIX Installer
1/25/2018 7:29:06 PM - -------------------------------------------
1/25/2018 7:29:06 PM - vsixinstaller.exe version:
1/25/2018 7:29:06 PM - 15.5.99+g87f89a485e
1/25/2018 7:29:06 PM - -------------------------------------------
1/25/2018 7:29:06 PM - Command line parameters:
1/25/2018 7:29:06 PM - C:\Program Files (x86)\Microsoft Visual Studio\Installer\resources\app\ServiceHub\Services\Microsoft.VisualStudio.Setup.Service\VSIXInstaller.exe,C:\Users\ijnji\Desktop\WebSharper.4.1.4.153.vsix
1/25/2018 7:29:06 PM - -------------------------------------------
1/25/2018 7:29:06 PM - Microsoft VSIX Installer
1/25/2018 7:29:06 PM - -------------------------------------------
1/25/2018 7:29:06 PM - Initializing Install...
1/25/2018 7:29:06 PM - Microsoft.VisualStudio.ExtensionManager.InvalidExtensionPackageException: The file is not a valid VSIX package.
at Microsoft.VisualStudio.ExtensionManager.InstallableExtensionImpl.ReadVSIXManifestFromPackage(Stream stream, CultureInfo preferredCulture)
at Microsoft.VisualStudio.ExtensionManager.InstallableExtensionImpl..ctor(String path, CultureInfo preferredCulture)
at Microsoft.VisualStudio.ExtensionManager.ExtensionEngineImpl.CreateInstallableExtension(String extensionPath, CultureInfo preferredCulture)
at Microsoft.VisualStudio.ExtensionManager.ExtensionEngineImpl.CreateInstallableExtension(String extensionPath)
at VSIXInstaller.App.GetInstallableData(String vsixPath, Boolean isRepairSupported, IEnumerable`1& skuData)
at VSIXInstaller.App.Initialize(Boolean isRepairSupported)
at VSIXInstaller.App.Initialize()
at System.Threading.Tasks.Task`1.InnerInvoke()
at System.Threading.Tasks.Task.Execute()