diff --git a/core/modules/system/system.js b/core/modules/system/system.js
index a8e8dc9..e66f244 100644
--- a/core/modules/system/system.js
+++ b/core/modules/system/system.js
@@ -9,23 +9,32 @@
  */
 Drupal.behaviors.copyFieldValue = {
   attach: function (context, settings) {
-    function updateTarget (e) {
-      var targetValue = $(e.target).val();
-      // Get the list of target fields.
-      var targetIds = settings.copyFieldValue[sourceId];
-      for (var delta in targetIds) {
-        if (targetIds.hasOwnProperty(delta)) {
-          var targetField = $('#' + targetIds[delta]);
-          if (targetField.val() === '') {
-            targetField.val(targetValue);
-          }
+    // Initialize source->target mapping or grab exisiting.
+    // This is basically a copy of settings but decoupled from the global scope.
+    this.map = this.map || {};
+    for (var sourceId in settings.copyFieldValue) {
+      if (settings.copyFieldValue.hasOwnProperty(sourceId)) {
+        var $source = $('#' + sourceId).once('copy-field-values');
+        if ($source.length) {
+          // Add the behavior to update target fields on blur of the primary
+          // field. Using $.proxy() so 'this' references the behavior and
+          // not the element triggering the handler.
+          $source.bind('blur', $.proxy(this.updateTarget, this));
+          this.map[sourceId] = settings.copyFieldValue[sourceId];
         }
       }
     }
-    for (var sourceId in settings.copyFieldValue) {
-      if (settings.copyFieldValue.hasOwnProperty(sourceId)) {
-        // Add the behavior to update target fields on blur of the primary field.
-        $('#' + sourceId).once('copy-field-values').bind('blur', updateTarget);
+  },
+  updateTarget: function (e) {
+    var targetValue = $(e.target).val();
+    // Get the list of target fields.
+    var targetIds = this.map[e.target.id];
+    for (var id in targetIds) {
+      if (targetIds.hasOwnProperty(id)) {
+        var $targetField = $('#' + targetIds[id]);
+        if ($targetField.val() === '') {
+          $targetField.val(targetValue);
+        }
       }
     }
   }