error potentially dangerous request.form value was detected from the client

I have a tinyMCE text editor on my page which is populated with content
which is already stored in the database as html.

eg. <p>first paragraph</p>
<p>second paragraph</p> etc, etc with no problems there.

but when I make a change in the editor and then try to update the content in the database
 I get the error potentially dangerous request.form value was detected from the client