ZML Button behaviour in HTML page

ZML Button behaviour in HTML page

I'm using a button in a html page to open a form and pre-populate it with data. I can get it to work except for when using a specific field - and I can see no reason why.

I'm building a Forum with posts and comments. I have a html page that is constructed to show as specific post using the parameter POSTID, when pressing the zml button it should open the comment form in a pop up and pre-populate it.
This is my ZML for the button
  1. <%{
  2.     varLookupID = Post[post_parameter_ID == POSTID].ID;
  3.     varLookupTitle = Post[post_parameter_ID == POSTID].post_title;
  4.     %>
  5. <panel>
  6.   <pr height='fill' width='fill'>
  7.     <pc width='100%' bgColor='#FFFFFF' padding='20px' vAlign='middle'>
  8.       <pr height='auto' width='auto'>
  9.         <pc>
  10.           <button text='Add Comment' size='15px' color='#ffffff' bgColor='#0072f4' type='flat' marginLeft='0px' marginRight='0px' marginBottom='0px' marginTop='0px' action='OpenForm' parameters='comment_postID=<%=varLookupID%>&amp;&amp;comment_post_title_only=<%=varLookupTitle%>' target='popup' popupWidth='1000px' popupHeight='600px' componentLinkName='comment' />
  11.         </pc>
  12.       </pr>
  13.     </pc>
  14.   </pr>
  15. </panel>
  16. <%
  17. }%>
I get the error :
Error details:
Error in ZML snippet - zml_snippet in the page View Post"
Line Number : 6
Element type button must be followed by either attribute specifications, ">" or "/>".

However when I change the  varLookupTitle to any other field then it works with no error. 
      varLookupTitle = Post[post_parameter_ID == POSTID].post_date; works
      varLookupTitle = Post[post_parameter_ID == POSTID].post_content; works
      varLookupTitle = Post[post_parameter_ID == POSTID].post_title; doesn't work

The post_title field is a single line text field with nothing special about it. I can not work out why this one field causes the error