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 27527a3..88cb8c5 100644
--- a/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java
+++ b/adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java
@@ -275,6 +275,9 @@ 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 ffc479e..37a3b1c 100644
--- a/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java
+++ b/adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java
@@ -105,6 +105,24 @@ 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")