diff --git a/morituri/test/common.py b/morituri/test/common.py
index d1aa8d7..97e716e 100644
--- a/morituri/test/common.py
+++ b/morituri/test/common.py
@@ -26,11 +26,13 @@ def _diff(old, new, desc):
 
     raise AssertionError(
         ("\nError while comparing strings:\n"
-         "%s") % (output, ))
+         "%s") % (output.encode('utf-8'), ))
 
 
 def diffStrings(orig, new, desc='input'):
 
+    assert type(orig) == type(new)
+
     def _tolines(s):
         return [line + '\n' for line in s.split('\n')]