diff --git a/src/main/java/com/google/devtools/build/lib/actions/HasDigest.java b/src/main/java/com/google/devtools/build/lib/actions/HasDigest.java
index e17d3953..68e5ab4 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/HasDigest.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/HasDigest.java
@@ -14,9 +14,10 @@
 package com.google.devtools.build.lib.actions;
 
 import com.google.protobuf.ByteString;
+import java.io.Serializable;
 
 /** A marker interface for objects which can return a byte[] digest. */
-public interface HasDigest {
+public interface HasDigest extends Serializable {
   byte[] getDigest();
 
   public static final HasDigest EMPTY = new ByteStringDigest(new byte[] {});