--- a/tests/run-tests.py	Mon Aug 17 10:59:30 2020 +0200
+++ b/tests/run-tests.py	Fri Sep 11 13:04:05 2020 +0530
@@ -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):