You’re modifying document while it’s being loaded (when browser hasn’t “seen” closing tag for this element) . This causes very tricky situation in the parser and in IE it’s not allowed.
IE blog has explanation of this.
The solution is to modify another element that’s earlier in the document and has been loaded completely (where browser already saw closing tag for it).
BTW: The string </ is not allowed in <script> element. Use <\/ which is a safe equivalent in JS strings.