retrieving count of all opportunities created by a user not working

<fetch version="1.0" output-format="xml-platform" mapping="logical" aggregate="true">
  <entity name="systemuser">
    <attribute name="fullname" />
    <attribute name="systemuserid" />
    <order attribute="fullname" descending="false" />
    <filter type="and">
      <condition attribute="isdisabled" operator="eq" value="0" />
    </filter>
    <link-entity name="opportunity" from="createdby" to="systemuserid" alias="ak" link-type="outer">
    <attribute name="opportunityid" alias="count" aggregate="count" />
      <filter type="and">
        <condition attribute="createdon" operator="last-month" />
      </filter>
    </link-entity>
  </entity>
</fetch>