diff --git a/core/api/src/test/java/org/onosproject/net/config/NetworkConfigServiceAdapter.java b/core/api/src/test/java/org/onosproject/net/config/NetworkConfigServiceAdapter.java
index 562fe5c..0553a2b 100644
--- a/core/api/src/test/java/org/onosproject/net/config/NetworkConfigServiceAdapter.java
+++ b/core/api/src/test/java/org/onosproject/net/config/NetworkConfigServiceAdapter.java
@@ -16,6 +16,7 @@
 package org.onosproject.net.config;
 
 import com.fasterxml.jackson.databind.JsonNode;
+import com.google.common.collect.ImmutableSet;
 
 import java.util.Set;
 
@@ -25,7 +26,7 @@
 public class NetworkConfigServiceAdapter implements NetworkConfigService {
     @Override
     public Set<Class> getSubjectClasses() {
-        return null;
+        return ImmutableSet.of();
     }
 
     @Override
@@ -45,17 +46,17 @@
 
     @Override
     public <S> Set<S> getSubjects(Class<S> subjectClass) {
-        return null;
+        return ImmutableSet.of();
     }
 
     @Override
     public <S, C extends Config<S>> Set<S> getSubjects(Class<S> subjectClass, Class<C> configClass) {
-        return null;
+        return ImmutableSet.of();
     }
 
     @Override
     public <S> Set<? extends Config<S>> getConfigs(S subject) {
-        return null;
+        return ImmutableSet.of();
     }
 
     @Override