diff --git a/twirp/base.py b/twirp/base.py
index 6417206..e7c867a 100644
--- a/twirp/base.py
+++ b/twirp/base.py
@@ -91,7 +91,7 @@ def proto_encoder(value, data_obj=None):
         return value.SerializeToString(), {"Content-Type": "application/protobuf"}
 
     def _get_encoder_decoder(self, endpoint, headers):
-        ctype = headers.get('content-type', None)
+        ctype = headers.get('content-type', '')
         if "application/json" == ctype:
             decoder = functools.partial(self.json_decoder, data_obj=endpoint.input)
             encoder = functools.partial(self.json_encoder, data_obj=endpoint.output)