karma stop when error object is not string (Firefox & IE)

[nam-truong-le]

I started a test with Karma and received the error TypeError: undefined is not a function. I tried to debug it and found out that: in the function createErrorFormatter in the file reporter.js this line couldn't be executed:
msg = (msg || '').replace(URL_REGEXP, function (_, prefix, path, __, ___, line, ____, column) {...

The reason is the msg which was given in the method onKarmaError in the file browser.js is not a String and hence the replace function is undefined:

Surprisingly this issue occurs only when I test with Firefox and IE. There is no issue with Chrome and Safari.