memoizeit

diff --git a/src/main/java/br/ufrgs/inf/prosoft/memoizeit/Occurrence.java b/src/main/java/br/ufrgs/inf/prosoft/memoizeit/Occurrence.java
index 385bee1..93c66e7 100644
--- a/src/main/java/br/ufrgs/inf/prosoft/memoizeit/Occurrence.java
+++ b/src/main/java/br/ufrgs/inf/prosoft/memoizeit/Occurrence.java
@@ -9,6 +9,7 @@ import br.ufrgs.inf.prosoft.cache.Cache;
 import br.ufrgs.inf.prosoft.cache.KeyNotFoundException;
 import br.ufrgs.inf.prosoft.memoizeit.graph.Node;
 import java.util.List;
+import java.util.Objects;
 
 /**
  *
@@ -58,7 +59,7 @@ public abstract class Occurrence {
         String key = getParameters().toString();
         try {
             Object cached = cache.get(key);
-            if (!cached.equals(getReturnValue())) {
+            if (!Objects.equals(cached, getReturnValue())) {
                 cache.invalidate(key);
             }
         } catch (KeyNotFoundException ex) {