diff --git a/wp-includes/js/customize-preview-widgets.js b/wp-includes/js/customize-preview-widgets.js
index 6bd7e02464..cccc5962f8 100644
--- a/wp-includes/js/customize-preview-widgets.js
+++ b/wp-includes/js/customize-preview-widgets.js
@@ -50,7 +50,14 @@
 
 				emptyWidget = $(widgetTpl);
 				widgetSelector = emptyWidget.prop('tagName');
-				widgetClasses = emptyWidget.prop('className').replace(/^\s+|\s+$/g, '');
+				widgetClasses = emptyWidget.prop('className');
+
+				// Prevent a rare case when before_widget, before_title, after_title and after_widget is empty.
+				if ( ! widgetClasses ) {
+					return;
+				}
+
+				widgetClasses = widgetClasses.replace(/^\s+|\s+$/g, '');
 
 				if ( widgetClasses ) {
 					widgetSelector += '.' + widgetClasses.split(/\s+/).join('.');
diff --git a/wp-includes/js/customize-preview-widgets.min.js b/wp-includes/js/customize-preview-widgets.min.js
index 85480c4957..890f5b5487 100644
--- a/wp-includes/js/customize-preview-widgets.min.js
+++ b/wp-includes/js/customize-preview-widgets.min.js
@@ -1 +1 @@
-!function(a,b){if(a&&a.customize){var c,d=a.customize;d.WidgetCustomizerPreview={renderedSidebars:{},renderedWidgets:{},registeredSidebars:[],registeredWidgets:{},widgetSelectors:[],preview:null,l10n:{},init:function(){var a=this;this.buildWidgetSelectors(),this.highlightControls(),this.preview.bind("active",function(){a.preview.send("rendered-sidebars",a.renderedSidebars),a.preview.send("rendered-widgets",a.renderedWidgets)}),this.preview.bind("highlight-widget",a.highlightWidget)},buildWidgetSelectors:function(){var a=this;b.each(this.registeredSidebars,function(c,d){var e,f,g,h=[d.before_widget.replace("%1$s","").replace("%2$s",""),d.before_title,d.after_title,d.after_widget].join("");e=b(h),f=e.prop("tagName"),g=e.prop("className").replace(/^\s+|\s+$/g,""),g&&(f+="."+g.split(/\s+/).join(".")),a.widgetSelectors.push(f)})},highlightWidget:function(a){var c=b(document.body),d=b("#"+a);c.find(".widget-customizer-highlighted-widget").removeClass("widget-customizer-highlighted-widget"),d.addClass("widget-customizer-highlighted-widget"),setTimeout(function(){d.removeClass("widget-customizer-highlighted-widget")},500)},highlightControls:function(){var a=this,c=this.widgetSelectors.join(",");b(c).attr("title",this.l10n.widgetTooltip),b(document).on("mouseenter",c,function(){a.preview.send("highlight-widget-control",b(this).prop("id"))}),b(document).on("click",c,function(c){c.shiftKey&&(c.preventDefault(),a.preview.send("focus-widget-control",b(this).prop("id")))})}},c=d.Preview,d.Preview=c.extend({initialize:function(a,b){d.WidgetCustomizerPreview.preview=this,c.prototype.initialize.call(this,a,b)}}),b(function(){var a=window._wpWidgetCustomizerPreviewSettings;a&&(b.extend(d.WidgetCustomizerPreview,a),d.WidgetCustomizerPreview.init())})}}(window.wp,jQuery);
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+!function(a,b){if(a&&a.customize){var c,d=a.customize;d.WidgetCustomizerPreview={renderedSidebars:{},renderedWidgets:{},registeredSidebars:[],registeredWidgets:{},widgetSelectors:[],preview:null,l10n:{},init:function(){var a=this;this.buildWidgetSelectors(),this.highlightControls(),this.preview.bind("active",function(){a.preview.send("rendered-sidebars",a.renderedSidebars),a.preview.send("rendered-widgets",a.renderedWidgets)}),this.preview.bind("highlight-widget",a.highlightWidget)},buildWidgetSelectors:function(){var a=this;b.each(this.registeredSidebars,function(c,d){var e,f,g,h=[d.before_widget.replace("%1$s","").replace("%2$s",""),d.before_title,d.after_title,d.after_widget].join("");e=b(h),f=e.prop("tagName"),g=e.prop("className"),g&&(g=g.replace(/^\s+|\s+$/g,""),g&&(f+="."+g.split(/\s+/).join(".")),a.widgetSelectors.push(f))})},highlightWidget:function(a){var c=b(document.body),d=b("#"+a);c.find(".widget-customizer-highlighted-widget").removeClass("widget-customizer-highlighted-widget"),d.addClass("widget-customizer-highlighted-widget"),setTimeout(function(){d.removeClass("widget-customizer-highlighted-widget")},500)},highlightControls:function(){var a=this,c=this.widgetSelectors.join(",");b(c).attr("title",this.l10n.widgetTooltip),b(document).on("mouseenter",c,function(){a.preview.send("highlight-widget-control",b(this).prop("id"))}),b(document).on("click",c,function(c){c.shiftKey&&(c.preventDefault(),a.preview.send("focus-widget-control",b(this).prop("id")))})}},c=d.Preview,d.Preview=c.extend({initialize:function(a,b){d.WidgetCustomizerPreview.preview=this,c.prototype.initialize.call(this,a,b)}}),b(function(){var a=window._wpWidgetCustomizerPreviewSettings;a&&(b.extend(d.WidgetCustomizerPreview,a),d.WidgetCustomizerPreview.init())})}}(window.wp,jQuery);
\ No newline at end of file