Why do I get a blank image when I set the KeywordFilter property of the AdRotator

If the KeywordFilter property of the AdRotator control is set to a keyword that does not correspond to any element in the Advertisement file then you may get a blank image.
Here is some code that should work properly.

<asp:AdRotator id='AdRotator1' AdvertisementFile='adrot.xml' 
	runat='server' Width='468px' KeywordFilter ='book' Height='60px'></asp:AdRotator>

adrot.xml

<Advertisements>  
  <Ad>
	<ImageUrl>b2346.jpg</ImageUrl>
	<NavigateUrl>http://ww.syncfusion.com</NavigateUrl>
	<AlternateText>
	         The site for ASP.Net FAQs
	</AlternateText>
	<Impressions>5</Impressions>
	<Keyword>book</Keyword>
   </Ad>
</Advertisements>