diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -3347,7 +3347,8 @@
         else:
             errpath = b'%s.err' % test['path']
         if self.options.outputdir:
-            errpath = os.path.join(self.options.outputdir, errpath)
+            self._outputdir = canonpath(_sys2bytes(self.options.outputdir))
+            errpath = os.path.join(self._outputdir, errpath)
         return errpath
 
     def _getport(self, count):