diff --git a/src/com/google/common/io/protocol/ProtoBuf.java b/src/com/google/common/io/protocol/ProtoBuf.java
index ff7a324380..2a2f8b7739 100644
--- a/src/com/google/common/io/protocol/ProtoBuf.java
+++ b/src/com/google/common/io/protocol/ProtoBuf.java
@@ -868,6 +868,7 @@ public class ProtoBuf {
         case ProtoBufType.TYPE_MESSAGE:
         case ProtoBufType.TYPE_TEXT:
         case ProtoBufType.TYPE_BYTES:
+        case ProtoBufType.TYPE_STRING:
           return;
       }
     } else if (object instanceof ProtoBuf) {