diff --git a/modoboa/core/management/commands/clean_unknown_extensions.py b/modoboa/core/management/commands/clean_unknown_extensions.py
index 5cc5cbcf0..12efa29fe 100644
--- a/modoboa/core/management/commands/clean_unknown_extensions.py
+++ b/modoboa/core/management/commands/clean_unknown_extensions.py
@@ -19,6 +19,6 @@ class Command(BaseCommand):
     def handle(self, *args, **options):
         """Entry point."""
         for extension in Extension.objects.all():
-            fullname = "modoboa.extensions.{}".format(extension.name)
+            fullname = "modoboa.extensions.{0}".format(extension.name)
             if not fullname in settings.MODOBOA_APPS:
                 extension.delete()
diff --git a/modoboa/core/management/commands/manage_extensions.py b/modoboa/core/management/commands/manage_extensions.py
index 0806d5ba0..2fa71b847 100644
--- a/modoboa/core/management/commands/manage_extensions.py
+++ b/modoboa/core/management/commands/manage_extensions.py
@@ -51,7 +51,7 @@ class Command(BaseCommand):
             return
 
         for extname in args:
-            fullname = "modoboa.extensions.{}".format(extname)
+            fullname = "modoboa.extensions.{0}".format(extname)
             if not fullname in settings.MODOBOA_APPS:
                 continue
             self._change_extension_state(extname, options)