Microsoft.Web.Administration.SslFlag TypeNotFound Error

Question

I have written a PowerShell module that contains a function with the following PARAM block.

    PARAM(
        [parameter(Mandatory = $true)]
        [string]                                    $ServerName,
        
        [parameter(Mandatory = $true)]
        [string]                                    $SiteName,

        [parameter(Mandatory = $true)]
        [string]                                    $CertificateName,

        [parameter(Mandatory = $true)]
        [string]                                    $HostName,

        [Microsoft.Web.Administration.SslFlags]     $SslFlags
    )

Sometimes when I call the function (of course, I import the PowerShell module it is in first) from a PowerShell script, it works fine. Other times it produces this runtime error.

Unable to find type [Microsoft.Web.Administration.SslFlags].
At <path_to_module>:877 char:9
+         [Microsoft.Web.Administration.SslFlags]                          $SslFlags
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Microsoft.Web.Administration.SslFlags:TypeName) [], RuntimeException
    + FullyQualifiedErrorId : TypeNotFound

When that error occurs, it is always preceded by this error in the script.

Unable to find type [Microsoft.Web.Administration.SslFlags].
At <path_to_script>:110 char:13
+ $sslFlags = [Microsoft.Web.Administration.SslFlags]::Sni
+             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Microsoft.Web.Administration.SslFlags:TypeName) [], RuntimeException
    + FullyQualifiedErrorId : TypeNotFound

What I am doing on line 110 of my script is attempting to set the value of a variable thusly:

$sslFlags = [Microsoft.Web.Administration.SslFlags]::Sni

What am I doing wrong? I've tried Google & Bing searches, but the results never address these errors that I only sometimes get.