Fixed "can't modify frozen String (RuntimeError)" error

See
https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-ContentTranslation-language-stage.wmflabs.org-linux-firefox/18/consoleFull

Change-Id: I079be2ca4a53d0e86fe6158875dbec407eb11fdc
Merge "Fixed "can't modify frozen String (RuntimeError)" error"