Tomcat, Java & SQL Server 2008 R2: Cannot create JDBC driver of class '' for connect URL 'null'

I am using Tomcat and Java (through Eclipse) and SQL Server 2008 R2

I am getting

org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of 
class '' for connect URL 'null'

I also tried deploying to Tomcat 7 and I got exactly the same error. (I made sure that in this case the sqljdbc4.jar was in the lib directory of Tomcat, rather than just in the WAR)

Can anyone help? Thanks!

My code is as follows:

Server.xml

<GlobalNamingResources>
<Resource type="javax.sql.DataSource" auth="Container"
        name="jdbc/charmDB"
        factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
        driverClassName="com.microsoft.sqlserver.jdbc.SQLServerDriver"
        url="jdbc:sqlserver://localhost:1433;databaseName=CH;
        integratedSecurity=true"
        />

Web.xml

<resource-ref>
  <description>CH Database</description>
  <res-ref-name>jdbc/charmDB</res-ref-name>
  <res-type>javax.sql.DataSource</res-type>
  <res-auth>Container</res-auth>
</resource-ref> 

Java code

InitialContext ic = new InitialContext();
DataSource ds = (DataSource) ic.lookup("java:comp/env/jdbc/charmDB");
Connection con = ds.getConnection();