getParent of emptyEnv returns null

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/env/REnvironment.java

@@ -1051,6 +1051,11 @@ public abstract class REnvironment extends RAttributeStorage implements RTypedVa
             super(EMPTY_ENV_NAME, new REnvEmptyFrameAccess());
         }
 
+        @Override
+        public REnvironment getParent() {
+            return null;
+        }
+
         @Override
         public void put(String key, Object value) throws PutException {
             throw new PutException(RError.Message.ENV_ASSIGN_EMPTY);