diff --git a/tests/modeltests/custom_pk/fields.py b/tests/modeltests/custom_pk/fields.py
index 2494d5fd2c..92096a0ff7 100644
--- a/tests/modeltests/custom_pk/fields.py
+++ b/tests/modeltests/custom_pk/fields.py
@@ -31,7 +31,7 @@ class MyAutoField(six.with_metaclass(models.SubfieldBase, models.CharField)):
     def pre_save(self, instance, add):
         value = getattr(instance, self.attname, None)
         if not value:
-            value = MyWrapper(''.join(random.sample(string.lowercase, 10)))
+            value = MyWrapper(''.join(random.sample(string.ascii_lowercase, 10)))
             setattr(instance, self.attname, value)
         return value