diff --git a/xmlrunner/result.py b/xmlrunner/result.py
index 1b7cce6..62b97b6 100644
--- a/xmlrunner/result.py
+++ b/xmlrunner/result.py
@@ -693,6 +693,8 @@ def _exc_info_to_string(self, err, test):
                 msgLines.append(STDERR_LINE % error)
         # This is the extra magic to make sure all lines are str
         encoding = getattr(sys.stdout, 'encoding', 'utf-8')
+        if encoding is None:
+            encoding = 'utf-8'
         lines = []
         for line in msgLines:
             if not isinstance(line, str):