diff --git a/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java b/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java
index 88cb8c5..27527a3 100644
--- a/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java
+++ b/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java
@@ -275,9 +275,6 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
     }
 
     @Override public void toJson(JsonWriter writer, Object value) throws IOException {
-      if (defaultValueSet && value == defaultValue) {
-        throw new IllegalArgumentException("Cannot serialize default value instance: " + value);
-      }
       Class<?> type = value.getClass();
       int labelIndex = subtypes.indexOf(type);
       if (labelIndex == -1) {
diff --git a/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java b/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java
index 37a3b1c..ffc479e 100644
--- a/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java
+++ b/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java
@@ -105,24 +105,6 @@ public final class PolymorphicJsonAdapterFactoryTest {
     assertThat(message).isNull();
   }
 
-  @Test public void toJsonDefaultValue() {
-    Error fallbackError = new Error(Collections.<String, Object>emptyMap());
-    Moshi moshi = new Moshi.Builder()
-        .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")
-            .withSubtype(Success.class, "success")
-            .withSubtype(Error.class, "error")
-            .withDefaultValue(fallbackError))
-        .build();
-    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
-
-    try {
-      String json = adapter.toJson(fallbackError);
-      fail();
-    } catch (IllegalArgumentException e) {
-      assertThat(e).hasMessageContaining("Cannot serialize default value instance");
-    }
-  }
-
   @Test public void unregisteredSubtype() {
     Moshi moshi = new Moshi.Builder()
         .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")