Fix assertions
diff --git a/cvp_checks/tests/test_mtu.py b/cvp_checks/tests/test_mtu.py
index 735091f..940c145 100644
--- a/cvp_checks/tests/test_mtu.py
+++ b/cvp_checks/tests/test_mtu.py
@@ -62,5 +62,5 @@
             row.insert(0, interf)
             mtu_data.append(row)
     assert len(mtu_data) == 0, \
-        "Several problems found: {1}".format(
+        "Several problems found: {0}".format(
         json.dumps(mtu_data, indent=4))