diff --git a/main.py b/main.py
index 747956b..9569bc8 100755
--- a/main.py
+++ b/main.py
@@ -23,7 +23,7 @@ config_schema = Schema({
         'endpoint': Use(str),
     },
     'zone': Use(str),
-    Optional('subdomain'): Use(str),
+    Optional('subdomain', default=None): Use(lambda s: str(s) if s is not None else None),
     Optional('field_type', default='A'): And(Use(str), Use(str.upper), dns.rdatatype.RdataType.make),
     Optional('logging', default={'level': ':WARNING,ovhdns:INFO'}): {
         Optional('level', default=':WARNING,ovhdns:INFO'): Use(str),