diff --git a/testing/marionette/client/marionette/runtests.py b/testing/marionette/client/marionette/runtests.py
index 20431763d99..2641c78ede0 100644
--- a/testing/marionette/client/marionette/runtests.py
+++ b/testing/marionette/client/marionette/runtests.py
@@ -66,7 +66,7 @@ class MarionetteTestResult(unittest._TextTestResult):
                     return
                 self.stream.writeln('START LOG:')
                 for line in testcase.loglines:
-                    self.stream.writeln(' '.join(line))
+                    self.stream.writeln(' '.join(line).encode('ascii', 'replace'))
                 self.stream.writeln('END LOG:')
 
     def getPerfData(self, test):