Unable to find type [System.Web.HttpUtility]

[alan-null]

Invoke-SlackAPI : Unable to find type [System.Web.HttpUtility].
At C:\Users\VssAdministrator\Documents\WindowsPowerShell\Modules\Slack.Backup\1.0.0\Public\api\Get-Channel.ps1:19 
char:9
+         Invoke-SlackAPI -Method 'conversations.list' -Token $Token -P ...
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Web.HttpUtility:TypeName) [Invoke-SlackAPI], RuntimeException
    + FullyQualifiedErrorId : TypeNotFound,Invoke-SlackAPI