diff --git a/src/main/java/com/bernardomg/tabletop/dice/visitor/RollTransformer.java b/src/main/java/com/bernardomg/tabletop/dice/visitor/RollTransformer.java
index <HASH>..<HASH> 100644
--- a/src/main/java/com/bernardomg/tabletop/dice/visitor/RollTransformer.java
+++ b/src/main/java/com/bernardomg/tabletop/dice/visitor/RollTransformer.java
@@ -3,6 +3,7 @@ package com.bernardomg.tabletop.dice.visitor;
 
 import com.bernardomg.tabletop.dice.history.RollResult;
 
+@FunctionalInterface
 public interface RollTransformer {
 
     public RollResult transform(final RollResult result, final Integer index);
Added annotation to ensure it always can be mapped to a lambda