diff --git a/src/wp-includes/js/tinymce/plugins/wordpress/plugin.js b/src/wp-includes/js/tinymce/plugins/wordpress/plugin.js
index 014f3217ff..2f9283281f 100644
--- a/src/wp-includes/js/tinymce/plugins/wordpress/plugin.js
+++ b/src/wp-includes/js/tinymce/plugins/wordpress/plugin.js
@@ -1,7 +1,7 @@
 /* global getUserSetting, setUserSetting */
 ( function( tinymce ) {
 // Set the minimum value for the modals z-index higher than #wpadminbar (100000)
-if ( tinymce.ui.FloatPanel.zIndex < 100100 ) {
+if ( ! tinymce.ui.FloatPanel.zIndex || tinymce.ui.FloatPanel.zIndex < 100100 ) {
 	tinymce.ui.FloatPanel.zIndex = 100100;
 }