diff --git a/sumpy/assignment_collection.py b/sumpy/assignment_collection.py
index 96d078a0f55b5d34e5a08231e28b7f5d9e94f089..dadf3fc954e890c2ecc53716feb4857187a14d2b 100644
--- a/sumpy/assignment_collection.py
+++ b/sumpy/assignment_collection.py
@@ -145,7 +145,7 @@ class SymbolicAssignmentCollection(object):
         return result
 
     def add_assignment(self, name, expr, root_name=None, wrt_set=None):
-        assert isinstance(name, str)
+        assert isinstance(name, six.string_types)
         assert name not in self.assignments
 
         if wrt_set is None: