diff --git a/repo2docker/utils.py b/repo2docker/utils.py
index 2912fac9..7df5ae8d 100644
--- a/repo2docker/utils.py
+++ b/repo2docker/utils.py
@@ -5,7 +5,7 @@ import shutil
 import subprocess
 import sys
 
-from traitlets import Integer
+from traitlets import Integer, TraitError
 
 
 def execute_cmd(cmd, capture=False, **kwargs):