Array.insert doesn't work

[increpare]

a = ["a","b","c","a","b","c"];
a.insert(2,"X");
trace(a);

line 2 : RUNTIME ERROR (in initial run) Unknown error type in line 2: Error name: TypeError Javascript Stack: callMethod@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:2777:13 call@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:2187:28 fcall@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:2184:19 expr@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:1905:23 expr@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:1853:19 exprReturn@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:1766:20 execute@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:1762:25 scriptfound@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:3250:33 loadscript@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:3208:24 update@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:3151:24 update@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:2327:19 doupdate@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:1010:26 ontimer@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:991:16 f@http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:24083:247 http://localhost:8000/haxegonweb/bin/html5/bin/webthing.js:3411:9