is.error() does not recognise sub-classes of Error as errors

[bbusschots-mu]

I have custom error classes in my code that are properly sub-classed from Error using the very convenient es6-error NPM module. Errors generated with these sub-classes are recognised as instances of Error by JavaScript, i.e. myError instanceof Error evaluates to true. However, is.error() evaluates to false. Surely anything that gets true for instanceof Error should be recognised as an error by is.error()?