Workaround for CanonicaI URLs by Yoast (Yoast version 1.3 and below)

If you are using Yoast version 1.3 and below, you need to edit: 

 /app/code/community/Yoast/CanonicalUrl/Block/Head.php 

Change 

class Yoast_CanonicalUrl_Block_Head extends Mage_Page_Block_Html_Head 

to 

class Yoast_CanonicalUrl_Block_Head extends Fooman_Speedster_Block_Page_Html_Head