mailparse_msg_extract_part_file error

I have written a PHP (v5.1) app to parse a bunch of MIME files. It works for the vast majority, but sometimes I get this error: mailparse_msg_extract_part_file(): mbstring doesn't know how to decode plain transfer encoding! It appears to be when messages have Content-Type: text/plain; charset=us-ascii or Content-Transfer-Encoding: plain or a combination of the two. I've searched on the error message and only found a url that times out. Does anyone have any experience with this? The code in question is this (it's part of a function that gets the email body): foreach ($this->_sectiondata as $index => $data) { $contdisp = $this->_getHeaderData($index, 'content-disposition'); if ($contdisp == "" && $data['content-type'] != 'multipart/mixed') { $sec = mailparse_msg_get_part($this->_mimehandle, $index); ob_start(); /* extract the part from the message file and dump it to the output buffer */ mailparse_msg_extract_part_file($sec, $this->_filename); $contents = ob_get_contents(); ob_end_clean(); $this->_body = $contents; } } Thanks! Regards, Bruce