I'm getting 404 by using fetchxml in my API query

<fetch version="1.0" mapping="logical">
<entity name="pss_gridassets">
<attribute name="pss_gridassetsid" />
<attribute name="pss_name" />
<attribute name="createdon" />
<order attribute="pss_name" descending="false" />
<filter type="and">
<condition attribute="statecode" operator="eq" value="0" />
</filter>
</entity>
</fetch>

            var fetchXml = '<fetch version="1.0" mapping="logical"><entity name="pss_gridassets"><attribute name="pss_gridassetsid" /><attribute name="pss_name" /><attribute name="createdon" /><order attribute="pss_name" descending="false" /><filter type="and"><condition attribute="statecode" operator="eq" value="0" /></filter></entity></fetch>';
            var encodedFetchXml = encodeURI(fetchXml);

            var req = new XMLHttpRequest();
            req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v8.0/pss_gridassets?fetchXml=" + encodedFetchXml, true);
            req.setRequestHeader("Accept", "application/json");
            req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
            req.onreadystatechange = function () {

                if (this.readyState === 4) {

                    this.onreadystatechange = null;

                    if (this.status === 200) {

                    }
                }

            };
            req.send();