SCRIPT438: Object doesn't support property or method 'endsWith' on IE11

[radhyps]

Web site works fine in Chrome and Firefox but fails on IE 11.0.9600.18097. I get the following error:

SCRIPT438: Object doesn't support property or method 'endsWith'

Caused by this line of code in ext.axd:

parentToIndicator=this.indicatorEl.parent();parentIsIndicator=parentToIndicator.id!==undefined&&parentToIndicator.id.endsWith('-indicator');

I also attached a screenshot.

I am using:
*Visual Studio 2015 Pro
*.NET Framework 4.6.1 MVC5
*Nuget packages Ext.NET.MVC5 4.8.1 and Ext.NET.Utilities 2.5.0.

I understand this is related to ES6 browser support but my understanding is that Ext.NET 4.8.1 is compatible with IE11.