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

<%{
varLookupID = Post[post_parameter_ID == POSTID].ID;
varLookupTitle = Post[post_parameter_ID == POSTID].post_title;
%>
<panel>
<pr height='fill' width='fill'>
<pc width='100%' bgColor='#FFFFFF' padding='20px' vAlign='middle'>
<pr height='auto' width='auto'>
<pc>
<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%>&&comment_post_title_only=<%=varLookupTitle%>' target='popup' popupWidth='1000px' popupHeight='600px' componentLinkName='comment' />
</pc>
</pr>
</pc>
</pr>
</panel>
<%
}%>

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