diff --git a/compile.js b/compile.js
index e4e4007..c09ae25 100644
--- a/compile.js
+++ b/compile.js
@@ -101,7 +101,7 @@ function compileDest(ctx) {
     for (var i = 0; i < ctx._proto.fields.length; i++) {
         var field = ctx._proto.fields[i];
         props[field.name + ': ' + JSON.stringify(ctx._defaults[field.name])] = true;
-        if (field.oneof) props[field.oneof + ': null'] = true;
+        if (field.oneof) props[field.oneof + ': undefined'] = true;
     }
     return '{' + Object.keys(props).join(', ') + '}';
 }
@@ -295,7 +295,7 @@ function getDefaultValue(field, value) {
     case 'string':   return value || '';
     case 'bool':     return value === 'true';
     case 'map':      return {};
-    default:         return null;
+    default:         return undefined;
     }
 }
 
diff --git a/test/compile.test.js b/test/compile.test.js
index 4d7bd52..4021bf6 100644
--- a/test/compile.test.js
+++ b/test/compile.test.js
@@ -222,8 +222,8 @@ test('handles all implicit default values', function(t) {
     t.equals(data.id, 0);
     t.deepEqual(data.tags, []);
     t.deepEqual(data.numbers, []);
-    t.equals(data.bytes, null);
-    t.equals(data.custom, null);
+    t.equals(data.bytes, undefined);
+    t.equals(data.custom, undefined);
     t.deepEqual(data.types, []);
 
     t.end();
@@ -237,7 +237,7 @@ test('sets oneof field name', function(t) {
     Envelope.write({}, pbf);
     var data = Envelope.read(new Pbf(pbf.finish()));
 
-    t.equals(data.value, null);
+    t.equals(data.value, undefined);
     t.equals(data.id, 0);
 
     pbf = new Pbf();