diff --git a/wire-runtime/src/main/java/com/squareup/wire/internal/ImmutableList.java b/wire-runtime/src/main/java/com/squareup/wire/internal/ImmutableList.java
index 5974c247a2..9fe497d50f 100644
--- a/wire-runtime/src/main/java/com/squareup/wire/internal/ImmutableList.java
+++ b/wire-runtime/src/main/java/com/squareup/wire/internal/ImmutableList.java
@@ -24,7 +24,7 @@
 import java.util.RandomAccess;
 
 final class ImmutableList<T> extends AbstractList<T> implements RandomAccess, Serializable {
-  final List<T> list;
+  private final ArrayList<T> list;
 
   ImmutableList(List<T> list) {
     this.list = new ArrayList<>(list);
@@ -38,6 +38,10 @@ final class ImmutableList<T> extends AbstractList<T> implements RandomAccess, Se
     return list.get(i);
   }
 
+  @Override public Object[] toArray() {
+    return list.toArray(); // Optimizing for mutable copy by MutableOnWriteList.
+  }
+
   private Object writeReplace() throws ObjectStreamException {
     return Collections.unmodifiableList(list);
   }