Why given code throws exception (Jsoup)

[Ceffooria]

As it is said on so many forums that element is a special case of node in DOM.But I got an exception that violate this rule.It throws exception at statement, \[code\]elem.remove()\[/code\]. Here, ele is an element. \[code\]remove()\[/code\] is a function in Jsoup API, that removes nodes and their descendants from DOM.Exception :-\[code\][WARNING] java.lang.reflect.InvocationTargetException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:601) at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:297) at java.lang.Thread.run(Thread.java:722)Caused by: java.lang.IllegalArgumentException: Object must not be null at org.jsoup.helper.Validate.notNull(Validate.java:16) at org.jsoup.nodes.Node.remove(Node.java:266) at XXX.YYY.ZZZ.Template_Matching.Template_Matching.removeProductLister(Template_Matching.java:80) at XXX.YYY.ZZZ.Template_Matching.Template_Matching.main(Template_Matching.java:376) ... 6 more[INFO] ------------------------------------------------------------------------[INFO] BUILD FAILURE\[/code\]Code :-\[code\]public static void function(Document doc1, Document doc2, String tag) { //Checking for ULs Elements uls_1 = doc1.getElementsByTag(tag); Elements uls_2 = doc2.getElementsByTag(tag); for (Element elem1 : uls_1) { // Check if elem1 exists in DOM, If No, then continue for (Element elem2 : uls_2) { // Check if elem2 exists in DOM, If No, then continue // If id matches, remove them if ((!"".equals(elem1.id())) && (elem1.id().equals(elem2.id()))) { elem1.remove(); elem2.remove(); break; } } } }\[/code\]