diff --git a/modules/ve-mw/ui/inspectors/ve.ui.MWExtensionInspector.js b/modules/ve-mw/ui/inspectors/ve.ui.MWExtensionInspector.js
index b8f4e01f89..a4acb11fdc 100644
--- a/modules/ve-mw/ui/inspectors/ve.ui.MWExtensionInspector.js
+++ b/modules/ve-mw/ui/inspectors/ve.ui.MWExtensionInspector.js
@@ -193,5 +193,13 @@ ve.ui.MWExtensionInspector.prototype.getTeardownProcess = function ( data ) {
  * @param {Object} mwData MediaWiki data object
  */
 ve.ui.MWExtensionInspector.prototype.updateMwData = function ( mwData ) {
-	mwData.body.extsrc = this.whitespace[0] + this.input.getValue() + this.whitespace[1];
+	var tagName = mwData.name,
+		value = this.input.getValue();
+
+	// XML-like tags in wikitext are not actually XML and don't expect their contents to be escaped.
+	// This means that it is not possible for a tag '<foo>…</foo>' to contain the string '</foo>'.
+	// Prevent that by escaping the first angle bracket '<' to '&lt;'. (bug 57429)
+	value = value.replace( new RegExp( '<(/' + tagName + '\\s*>)', 'gi' ), '&lt;$1' );
+
+	mwData.body.extsrc = this.whitespace[0] + value + this.whitespace[1];
 };