No definition found for Table data.headers

[niutech]

When using the data.headers table like:
https://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20data.headers%20where%20url%3D%22http%3A%2F%2Fwww.yahoo.com%2F%22&format=json&diagnostics=true&debug=true&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&callback=
sometimes I get the 400 Bad request response like:

{"error":{"lang":"en-US","diagnostics":{"url":[{"execution-start-time":"1","execution-stop-time":"4","execution-time":"3","id":"83e9d910-8b74-4a64-84b0-68391d179ee9","content":"http://sherpa-bcp5903.dht.yahoo.com:4080/YDHTWebService/V1/get/yql.global/store%3A%2F%2Fdatatables.org%2Falltableswithkeys"},{"execution-start-time":"5","execution-stop-time":"14","execution-time":"9","id":"30dc0e33-b204-4df1-afee-d91edfd5b9db","content":"http://sherpa-bcp5903.dht.yahoo.com:4080/YDHTWebService/V1/get/yql.global/store%3A%2F%2FRjdEzitN2Hceujh3tGHPj6"},{"execution-start-time":"36","execution-stop-time":"45","execution-time":"9","id":"b7023b73-25f9-4622-9a5a-71fc033fd404","content":"http://sherpa-bcp5903.dht.yahoo.com:4080/YDHTWebService/V1/get/yql.global/store%3A%2F%2FRjdEzitN2Hceujh3tGHPj6"},{"execution-start-time":"68","execution-stop-time":"69","execution-time":"1","http-status-code":"400","http-status-message":"Bad Request","id":"18732518-eff5-4436-9346-25e561daa60b","content":"http://www.datatables.org/data/data.headers.xml"}]},"description":"No definition found for Table data.headers"}}

Why is that so?