Zend Mail - Getting extra characters from getBodyText

Question

I'm having an issue with Zend Mail. When I try to preppend some text to the body text before it's sent, I end up with extra characters, like '=' that shouldn't be there. When the email is sent normally, they don't appear either. Here's what I mean (this is from a class that extends Zend_Mail):

$bodyHtml = $this->getBodyHtml(true);
$this->setBodyHtml('Adding some new text here\n\n' . $bodyHtml);

When this gets sent, it ends up with something like:

Adding some new text here
Here's the origin=al text with these equ=al signs added

Any idea where these = signs are coming from?