title: Security.SecureString parameter does not accept strings
body: <p>When passing a string as an argument to this parameter:</p>
  
  <pre><code>[Parameter(Mandatory=$true)]
  [ValidateNotNullOrEmpty()]
  [Security.SecureString]$password=$(Throw "Password required.")
  </code></pre>
  
  <p>I get this error when using the <code>-password</code> parameter. </p>
  
  <blockquote>
    <p>cannot convert type System.String to type System.Security.SecureString </p>
  </blockquote>
  
  <p>If I don't pass the <code>-password</code> parameter, a prompt is shown and it accepts the input.</p>