Error in Google Apps Script: mime type error when using getPlainText

Error in Google Apps Script: mime type error when using getPlainText

When I tried to use the getPlainBody() method of GmailMessage class. When
I execute the following code, the error message "Invalid MIME type" show
up. Can anyone help me to verify what's wrong in my code?
function processLabel(labelName) {
var targetLabel = GmailApp.getUserLabelByName(labelName);
var targetThreads = targetLabel.getThreads();
return targetThreads;
};
function getEmailPlainText() {
var targetThreads = processLabel('@today');
var threadsCount = targetThreads.length;
for (var i = 0; i < threadsCount; i++) {
var targetThread = targetThreads[i];
var messageCount = targetThread.getMessageCount();
var subject = targetThread.getFirstMessageSubject();
var lastMessage = targetThread.getMessages()[messageCount-1];
var lastMessageContent = lastMessageContent = lastMessage.getPlainBody();
};