diff --git a/java/google/registry/model/eppcommon/ProtocolDefinition.java b/java/google/registry/model/eppcommon/ProtocolDefinition.java
index ec2d198e5..990682cb7 100644
--- a/java/google/registry/model/eppcommon/ProtocolDefinition.java
+++ b/java/google/registry/model/eppcommon/ProtocolDefinition.java
@@ -64,8 +64,8 @@ public class ProtocolDefinition {
 
     private final Class<? extends CommandExtension> commandExtensionClass;
     private final Class<? extends ResponseExtension> responseExtensionClass;
-    private String uri;
-    private boolean visible;
+    private final String uri;
+    private final boolean visible;
 
     ServiceExtension(
         Class<? extends CommandExtension> commandExtensionClass,