i don't know if is a bug,when change the oSettings.sAjaxSource,the next button is disabled?

i change the oSettings.sAjaxSource of one datatables instance ,the other button is fine,but the next button is disabled,when i change the source like below ,the next button is fine again.
else if (mAction == "next") {
if (oSettings._iDisplayLength >= 0) {
/* Make sure we are not over running the display array */
if (parseInt(oSettings._iDisplayStart) + parseInt(oSettings._iDisplayLength) < parseInt(oSettings.fnRecordsDisplay())) {
oSettings._iDisplayStart += parseInt(oSettings._iDisplayLength);
}
}
else {
oSettings._iDisplayStart = 0;
}
}