diff --git a/nipype/interfaces/traits_extension.py b/nipype/interfaces/traits_extension.py
index 5490b567f6..02342aa96a 100644
--- a/nipype/interfaces/traits_extension.py
+++ b/nipype/interfaces/traits_extension.py
@@ -33,7 +33,7 @@
 from traits.api import BaseUnicode
 from traits.api import Unicode
 
-DictStrStr = traits.Dict(str, (bytes, str))
+DictStrStr = traits.Dict((bytes, str), (bytes, str))
 Str = Unicode
 
 class BaseFile(BaseUnicode):