Polymer/docs — pull request #846 by chrisconover (2014-12-23 22:16:52+00:00)
Title: Check that value exists in filter functions

It seems like a lot of people are confused about why they're seeing the console error "Cannot read property 'toUpperCase' of undefined" when copying the example from the documentation.
http://stackoverflow.com/questions/23681366/polymer-custom-filters

It may be helpful to include a check that value is defined before doing any operation on it.