Zend framework Undefined class constant '' in Zend/Db/Select.php - database

Question

There is a srange error in my Apache Log :
PHP Fatal error: Undefined class constant ''
This is on line 814 of de library (Zend framework 1.11) library/Zend/Db/Select.php
A look of the code :
if ($type == self::FROM) {
// append this from after the last from joinType
$tmpFromParts = $this->_parts[self::FROM];
$this->_parts[self::FROM] = array();
// move all the froms onto the stack
while ($tmpFromParts) {
$currentCorrelationName = key($tmpFromParts);
if ($tmpFromParts[$currentCorrelationName]['joinType'] != self::FROM) {
break;
}
$lastFromCorrelationName = $currentCorrelationName;
$this->_parts[self::FROM][$currentCorrelationName] = array_shift($tmpFromParts);
} }
That's strange because the undefined constant is empty.
THis fatal error appears some times but not at every load of pages.
Is it a release BUG of the zend framework?