diff --git a/src/tiledimage.js b/src/tiledimage.js
index 3518d272..24583e4d 100644
--- a/src/tiledimage.js
+++ b/src/tiledimage.js
@@ -1013,6 +1013,9 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
      * TODO
      */
     setAjaxHeaders: function(ajaxHeaders, propagate){
+        if (propagate === undefined) {
+            propagate = true;
+        }
 
         // use same headers if provided 'ajaxHeaders' is invalid (useful for propagation)
         if ($.isPlainObject(ajaxHeaders)) {
diff --git a/src/viewer.js b/src/viewer.js
index 8e2eeaa1..75113dea 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -982,6 +982,10 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
      * TODO
      */
     setAjaxHeaders: function(ajaxHeaders, propagate){
+        if (propagate === undefined) {
+            propagate = true;
+        }
+
         if ($.isPlainObject(ajaxHeaders)) {
             this.ajaxHeaders = ajaxHeaders;
         }