diff --git a/value/src/it/functional/pom.xml b/value/src/it/functional/pom.xml
index 0a98befbfc..c1a4bf9d6f 100644
--- a/value/src/it/functional/pom.xml
+++ b/value/src/it/functional/pom.xml
@@ -43,6 +43,16 @@
       <artifactId>auto-value</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>com.google.auto.service</groupId>
+      <artifactId>auto-service</artifactId>
+      <version>1.0-rc4</version>
+    </dependency>
+    <dependency>
+      <groupId>com.google.guava</groupId>
+      <artifactId>guava</artifactId>
+      <version>23.5-jre</version>
+    </dependency>
     <dependency>
       <groupId>com.google.code.findbugs</groupId>
       <artifactId>jsr305</artifactId>
@@ -60,12 +70,6 @@
       <version>4.12</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>com.google.guava</groupId>
-      <artifactId>guava</artifactId>
-      <version>23.5-jre</version>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>com.google.guava</groupId>
       <artifactId>guava-testlib</artifactId>
diff --git a/value/src/it/functional/src/test/java/com/google/auto/value/gwt/EmptyExtension.java b/value/src/it/functional/src/test/java/com/google/auto/value/gwt/EmptyExtension.java
new file mode 100644
index 0000000000..f07c917108
--- /dev/null
+++ b/value/src/it/functional/src/test/java/com/google/auto/value/gwt/EmptyExtension.java
@@ -0,0 +1,135 @@
+/*
+ * Copyright (C) 2018 Google, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.auto.value.gwt;
+
+import static java.util.stream.Collectors.joining;
+
+import com.google.auto.service.AutoService;
+import com.google.auto.value.extension.AutoValueExtension;
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.escapevelocity.Template;
+import java.io.IOException;
+import java.io.StringReader;
+import java.util.List;
+import javax.lang.model.element.TypeElement;
+import javax.lang.model.element.TypeParameterElement;
+import javax.lang.model.type.TypeMirror;
+
+/**
+ * An AutoValue extension that generates a subclass that does nothing useful.
+ */
+@AutoService(AutoValueExtension.class)
+public class EmptyExtension extends AutoValueExtension {
+  // TODO(emcmanus): it is way too difficult to write a trivial extension. Problems we have here:
+  //   (1) We have to generate a constructor that calls the superclass constructor, which means
+  //       declaring the appropriate constructor parameters and then forwarding them to a super
+  //       call.
+  //   (2) We have to avoid generating variable names that are keywords (we append $ here
+  //       to avoid that).
+  //   (3) We have to concoct appropriate type parameter strings, for example
+  //       final class AutoValue_Foo<K extends Comparable<K>, V> extends $AutoValue_Foo<K, V>.
+  //   These problems show up with the template approach here, but also using JavaPoet as the
+  //   Memoize extension does.
+  private static final ImmutableList<String> TEMPLATE_LINES =
+      ImmutableList.of(
+          "package $package;",
+          "\n",
+          "#if ($isFinal) final #end class ${className}${formalTypes}"
+              + " extends ${classToExtend}${actualTypes} {\n",
+          "  ${className}(",
+          "    #foreach ($property in $properties.keySet())",
+          "    $properties[$property].returnType ${property}$ #if ($foreach.hasNext) , #end",
+          "    #end",
+          "  ) {",
+          "    super(",
+          "      #foreach ($property in $properties.keySet())",
+          "      ${property}$ #if ($foreach.hasNext) , #end",
+          "      #end",
+          "    );",
+          "  }",
+          "}");
+
+  @Override
+  public boolean applicable(Context context) {
+    return true;
+  }
+
+  @Override
+  public String generateClass(
+      Context context, String className, String classToExtend, boolean isFinal) {
+    String templateString = Joiner.on('\n').join(TEMPLATE_LINES);
+    StringReader templateReader = new StringReader(templateString);
+    Template template;
+    try {
+      template = Template.parseFrom(templateReader);
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+    TypeElement autoValueClass = context.autoValueClass();
+    ImmutableMap<String, Object> vars =
+        ImmutableMap.<String, Object>builder()
+            .put("package", context.packageName())
+            .put("className", className)
+            .put("classToExtend", classToExtend)
+            .put("isFinal", isFinal)
+            .put("properties", context.properties())
+            .put("formalTypes", formalTypeParametersString(autoValueClass))
+            .put("actualTypes", actualTypeParametersString(autoValueClass))
+            .build();
+    return template.evaluate(vars);
+  }
+
+  private static String actualTypeParametersString(TypeElement type) {
+    List<? extends TypeParameterElement> typeParameters = type.getTypeParameters();
+    if (typeParameters.isEmpty()) {
+      return "";
+    }
+    return typeParameters
+        .stream()
+        .map(e -> e.getSimpleName().toString())
+        .collect(joining(", ", "<", ">"));
+  }
+
+  private static String formalTypeParametersString(TypeElement type) {
+    List<? extends TypeParameterElement> typeParameters = type.getTypeParameters();
+    if (typeParameters.isEmpty()) {
+      return "";
+    }
+    StringBuilder sb = new StringBuilder("<");
+    String sep = "";
+    for (TypeParameterElement typeParameter : typeParameters) {
+      sb.append(sep);
+      sep = ", ";
+      appendTypeParameterWithBounds(typeParameter, sb);
+    }
+    return sb.append(">").toString();
+  }
+
+  private static void appendTypeParameterWithBounds(
+      TypeParameterElement typeParameter, StringBuilder sb) {
+    sb.append(typeParameter.getSimpleName());
+    String sep = " extends ";
+    for (TypeMirror bound : typeParameter.getBounds()) {
+      if (!bound.toString().equals("java.lang.Object")) {
+        sb.append(sep);
+        sep = " & ";
+        sb.append(bound);
+      }
+    }
+  }
+}
diff --git a/value/src/main/java/com/google/auto/value/processor/GwtSerialization.java b/value/src/main/java/com/google/auto/value/processor/GwtSerialization.java
index 1d64162dde..9bcf5614cc 100644
--- a/value/src/main/java/com/google/auto/value/processor/GwtSerialization.java
+++ b/value/src/main/java/com/google/auto/value/processor/GwtSerialization.java
@@ -83,7 +83,7 @@ void maybeWriteGwtSerializer(AutoValueTemplateVars autoVars) {
     if (shouldWriteGwtSerializer()) {
       GwtTemplateVars vars = new GwtTemplateVars();
       vars.pkg = autoVars.pkg;
-      vars.subclass = autoVars.subclass;
+      vars.subclass = autoVars.finalSubclass;
       vars.formalTypes = autoVars.formalTypes;
       vars.actualTypes = autoVars.actualTypes;
       vars.useBuilder = !autoVars.builderTypeName.isEmpty();