Problem with Print button on CMS page

[Palle]

Hi solvers, I have problem with putting a "Print button" on my CMS-pages. I get the error Property CMS->content is not valid at line 878 in file classes/ObjectModel.php

and at line 878 it says, 

877.                         throw new PrestaShopException($message);
878.                     return $error_return ? $message : false;

 

Funny thing is that, if I try it before saving, it works fine, but then, when I save the code I get the message.

 

The code I use is very simple.

 

<form>

Some text ......
<input type="button" id='winBtn' value="Print" onclick="window.print()">
</form>

 

I use Prestashop version: 1.5.6.2

 

And yes I have click "Accept Iframes".

 

Anyone who have a solution on this?