Index: /FCKeditor/trunk/editor/_source/fckeditorapi.js
===================================================================
--- /FCKeditor/trunk/editor/_source/fckeditorapi.js	(revision 172)
+++ /FCKeditor/trunk/editor/_source/fckeditorapi.js	(revision 173)
@@ -35,4 +35,8 @@
 		// available if the editor instance is removed ("Can't execute code
 		// from a freed script" error).
+
+		// Note: we check the existence of oEditor.GetParentForm because some external 
+		// code (like JSON) can extend the Object protype and we get then extra oEditor 
+		// objects that aren't really FCKeditor instances.
 		var sScript =
 			'var FCKeditorAPI = {' +
@@ -51,5 +55,5 @@
 					'{' +
 						'var oEditor = FCKeditorAPI.__Instances[ name ] ;' +
-						'if ( oEditor.GetParentForm() == this )' +
+						'if ( oEditor.GetParentForm && oEditor.GetParentForm() == this )' +
 							'oEditor.UpdateLinkedField() ;' +
 					'}' +