InPrivate failure on EDGE / IE 11

[aander07]

Upon loading https://gusclass.com/demos/button2/min.html in an InPrivate window, EDGE logs:

SCRIPT5007: Unable to get property 'open' of undefined or null reference
2552014555-idpiframe.js (53,13)

Javascript processing stops and no authentication window is displayed.

Microsoft Edge 38.14393.0.0
Microsoft EdgeHTML 14.14393

The exact same error is reported in IE 11, also using an InPrivate window:

SCRIPT5007: Unable to get property 'open' of undefined or null reference
2552014555-idpiframe.js (53,13)

Version: 11.576.14393.0
Update Version: 11.0.38

Both browsers work correctly when not using InPrivate.