diff --git a/Lib/trace.py b/Lib/trace.py
index 9ce240f..e6316b6 100644
--- a/Lib/trace.py
+++ b/Lib/trace.py
@@ -117,7 +117,7 @@
         self._mods = modules or []
         self._dirs = dirs or []
 
-        self._dirs = map(os.path.normpath, self._dirs)
+        self._dirs = list(map(os.path.normpath, self._dirs))
         self._ignore = { '<string>': 1 }
 
     def names(self, filename, modulename):