fixed internal error on unprintable raised AssertionErrors

_pytest/_code/code.py

@@ -352,6 +352,8 @@ class ExceptionInfo(object):
         help for navigating the traceback.
     """
     _striptext = ''
+    _assert_start_repr = "AssertionError(u\'assert " if sys.version_info[0] < 3 else "AssertionError(\'assert "
+
     def __init__(self, tup=None, exprinfo=None):
         import _pytest._code
         if tup is None:
@@ -359,8 +361,8 @@ class ExceptionInfo(object):
             if exprinfo is None and isinstance(tup[1], AssertionError):
                 exprinfo = getattr(tup[1], 'msg', None)
                 if exprinfo is None:
-                    exprinfo = py._builtin._totext(tup[1])
-                if exprinfo and exprinfo.startswith('assert '):
+                    exprinfo = py.io.saferepr(tup[1])
+                if exprinfo and exprinfo.startswith(self._assert_start_repr):
                     self._striptext = 'AssertionError: '
         self._excinfo = tup
         #: the exception class