Useless, but other methods do the same, and it's best for symmetry here, I believe.
Use Moshi's JsonAdapter.serializeNulls() method.
Once it's released.

https://github.com/square/moshi/pull/238
pom.xml retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java
pom.xml retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java
diff --git a/pom.xml b/pom.xml
index a2309e1769..4dd4aa6ab7 100644
--- a/pom.xml
+++ b/pom.xml
@@ -63,7 +63,7 @@
     <jackson.version>2.7.2</jackson.version>
     <wire.version>2.2.0</wire.version>
     <simplexml.version>2.7.1</simplexml.version>
-    <moshi.version>1.3.0</moshi.version>
+    <moshi.version>1.4.0</moshi.version>
 
     <!-- Sample Dependencies -->
     <jsoup.version>1.7.3</jsoup.version>
diff --git a/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java b/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java
index 0b95d19518..311d76d0dc 100644
--- a/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java
+++ b/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiConverterFactory.java
@@ -94,6 +94,9 @@ public MoshiConverterFactory withNullSerialization() {
     if (failOnUnknown) {
       adapter = adapter.failOnUnknown();
     }
+    if (serializeNulls) {
+      adapter = adapter.serializeNulls();
+    }
     return new MoshiResponseBodyConverter<>(adapter);
   }
 
@@ -106,7 +109,10 @@ public MoshiConverterFactory withNullSerialization() {
     if (failOnUnknown) {
       adapter = adapter.failOnUnknown();
     }
-    return new MoshiRequestBodyConverter<>(adapter, serializeNulls);
+    if (serializeNulls) {
+      adapter = adapter.serializeNulls();
+    }
+    return new MoshiRequestBodyConverter<>(adapter);
   }
 
   private static Set<? extends Annotation> jsonAnnotations(Annotation[] annotations) {
diff --git a/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java b/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java
index 5e59b17fed..edbcf243e5 100644
--- a/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java
+++ b/retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiRequestBodyConverter.java
@@ -27,17 +27,14 @@ final class MoshiRequestBodyConverter<T> implements Converter<T, RequestBody> {
   private static final MediaType MEDIA_TYPE = MediaType.parse("application/json; charset=UTF-8");
 
   private final JsonAdapter<T> adapter;
-  private final boolean serializeNulls;
 
-  MoshiRequestBodyConverter(JsonAdapter<T> adapter, boolean serializeNulls) {
+  MoshiRequestBodyConverter(JsonAdapter<T> adapter) {
     this.adapter = adapter;
-    this.serializeNulls = serializeNulls;
   }
 
   @Override public RequestBody convert(T value) throws IOException {
     Buffer buffer = new Buffer();
     JsonWriter writer = JsonWriter.of(buffer);
-    writer.setSerializeNulls(serializeNulls);
     adapter.toJson(writer, value);
     return RequestBody.create(MEDIA_TYPE, buffer.readByteString());
   }
