The closest method ends in the following loop:

for ( i = 0, l = this.length; i < l; i++ ) {

cur = this[i];

while ( cur ) {

if ( pos ? pos.index(cur) > -1 :

jQuery.find.matchesSelector(cur, selectors) ) {

ret.push( cur );

break;

} else {

cur = r.parentNode;

if (!cur.ownerDocument || cur === context ) {

break;

}

}

}

}

the following line:

if (!cur.ownerDocument || cur === context ) {

will not work if r.parentNode return null,

thus it should be changed to

if (cur && !cur.ownerDocument || cur === context ) {

Best regards,

Laurent