In some page, type 5 + 15 in chrome devtool console, the output is undefined

Rt, in https://raw.githubusercontent.com/domchristie/humps/master/humps.js, I type 5 + 15 in chrome devTool's Console pane, the output is undefined, I think the output should be 20 (In other web page, for example https://google.com, it is 20)

Figure 1 the scene

And I notice that this page use CSP (Content-Security-Policy) with http header's Content-Security-Policy is "default-src 'none'; style-src 'unsafe-inline'; sandbox"

I think the key maybe sandbox, but I change other page's http header with CSP with sandbox, it