diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 2c184b7314..7448c1346a 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -372,6 +372,7 @@ class RegisterRestServlet(RestServlet):
         """
         reqd = ('medium', 'address', 'validated_at')
         if any(x not in threepid for x in reqd):
+            # This will only happen if the ID server returns a malformed response
             logger.info("Can't add incomplete 3pid")
             return
 
@@ -437,6 +438,7 @@ class RegisterRestServlet(RestServlet):
         """
         reqd = ('medium', 'address', 'validated_at')
         if any(x not in threepid for x in reqd):
+            # This will only happen if the ID server returns a malformed response
             logger.info("Can't add incomplete 3pid")
             defer.returnValue()