XrmServiceToolkit RetrieveMultiple returning 404 Not Found error

I am using XrmServiceToolkit version 2.2.1 in CRM 2016 and anytime I call this code it's getting a 404 not found error. I used OData query designer and copied the filter and entity name directly from what was generated so the case is correct. Any ideas why this wouldn't work?

 var accountArray = new Array();

     XrmServiceToolkit.Rest.RetrieveMultiple(
                  "New_alt_account_numberSet",
                  "$select=New_AccountNumber&$filter=New_ClientAccountId/Id eq guid'"+sClientAccountGUID+"'",
                  function (results) {
                      if (results.length >= 1)
                      {
                          for (var i = 0; i < results.length; i++) {
                              accountArray.push(results[i]);
                          }
                      }
                      else
                          return null;
                  },
                  function (error) {
                      alert(error.message);
                  },
                  function onComplete() {
                      //alert(" records should have been retrieved.");
                  },
                  false
              );

     return accountArray.join(" | ");