Error download using pop3 c #: Mediatype must be in the format type \ "/ \" subtype \ r \ parameter name: mediaType

I'm trying to get an email with an attachment from a mail box using pop3, the error occurs when I get the message does anyone know why this happens ?. It only happens with some emails.

 for (int i = 1; i <= cantidadMesajes; i++)
 {
   mensaje = client.GetMessage(i);
 }

Error:

  

Message: Media Type must be in the format type "/" subtype

     

parameter name: mediaType