diff --git a/runtime/src/main/scala/com/soundcloud/twinagle/TwirpEndpointFilter.scala b/runtime/src/main/scala/com/soundcloud/twinagle/TwirpEndpointFilter.scala
index a24d07c..f89a362 100644
--- a/runtime/src/main/scala/com/soundcloud/twinagle/TwirpEndpointFilter.scala
+++ b/runtime/src/main/scala/com/soundcloud/twinagle/TwirpEndpointFilter.scala
@@ -4,7 +4,7 @@ import com.twitter.finagle.http.{MediaType, Request, Response, Status}
 import com.twitter.finagle.{Filter, Service}
 import com.twitter.io.Buf
 import com.twitter.util.Future
-import scalapb.json4s.JsonFormat
+import scalapb.json4s.{JsonFormat, Printer}
 import scalapb.{GeneratedMessage, GeneratedMessageCompanion}
 
 /**
@@ -17,6 +17,9 @@ private[twinagle] class TwirpEndpointFilter[
     Req <: GeneratedMessage: GeneratedMessageCompanion,
     Rep <: GeneratedMessage: GeneratedMessageCompanion
 ] extends Filter[Request, Response, Req, Rep] {
+
+  val printer = new Printer().includingDefaultValueFields
+
   override def apply(
       request: Request,
       service: Service[Req, Rep]
@@ -26,7 +29,7 @@ private[twinagle] class TwirpEndpointFilter[
       service(input).map { r =>
         val response = Response(Status.Ok)
         response.contentType = MediaType.Json
-        response.contentString = JsonFormat.toJsonString(r)
+        response.contentString = printer.print(r)
         response
       }
     case Some("application/protobuf") =>
diff --git a/runtime/src/test/protobuf/test.proto b/runtime/src/test/protobuf/test.proto
index 64a85b2..36b95b0 100644
--- a/runtime/src/test/protobuf/test.proto
+++ b/runtime/src/test/protobuf/test.proto
@@ -2,4 +2,8 @@ syntax = "proto3";
 
 package com.soundcloud.twinagle.test;
 
-message TestMessage {};
\ No newline at end of file
+message TestMessage {};
+
+message HasField {
+    int32 foo = 1;
+}
\ No newline at end of file
diff --git a/runtime/src/test/scala/com/soundcloud/twinagle/TwirpEndpointFilterSpec.scala b/runtime/src/test/scala/com/soundcloud/twinagle/TwirpEndpointFilterSpec.scala
index 91a3b3e..fa92174 100644
--- a/runtime/src/test/scala/com/soundcloud/twinagle/TwirpEndpointFilterSpec.scala
+++ b/runtime/src/test/scala/com/soundcloud/twinagle/TwirpEndpointFilterSpec.scala
@@ -1,6 +1,6 @@
 package com.soundcloud.twinagle
 
-import com.soundcloud.twinagle.test.TestMessage
+import com.soundcloud.twinagle.test.{HasField,TestMessage}
 import com.twitter.finagle.Service
 import com.twitter.finagle.http.{Request, Status}
 import com.twitter.util.{Await, Future, Throw}
@@ -57,6 +57,20 @@ class TwirpEndpointFilterSpec extends Specification {
         response.status ==== Status.Ok
         response.contentString ==== "{}"
       }
+      
+      "serializes default values" in {
+        val svc = new TwirpEndpointFilter[HasField, HasField] andThen
+          Service.mk[HasField, HasField](msg => Future.value(msg))
+
+        val request = Request()
+        request.contentType = "application/json; charset=UTF-8"
+        request.contentString = "{}"
+
+        val response = Await.result(svc(request))
+
+        response.status ==== Status.Ok
+        response.contentString ==== """{"foo":0}"""
+      }
     }
 
     "un-supported content-type" in new Context {
@@ -71,7 +85,7 @@ class TwirpEndpointFilterSpec extends Specification {
       }
     }
 
-    "unspecified" in new Context {
+    "unspecified content-type" in new Context {
       val request = Request()
       request.contentString = "{}"