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