diff --git a/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserPairBuilder.java b/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserPairBuilder.java
index bc5d0b8e8..5e809debe 100644
--- a/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserPairBuilder.java
+++ b/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserPairBuilder.java
@@ -32,6 +32,8 @@
 import org.incendo.cloud.parser.ArgumentParseResult;
 import org.incendo.cloud.type.tuple.Pair;
 
+import static java.util.Objects.requireNonNull;
+
 public final class AggregateParserPairBuilder<C, U, V, O> {
 
     /**
@@ -127,8 +129,9 @@ public AggregateParser<C, O> build() {
      * @param <O>    output type
      * @return mapper
      */
-    static <C, U, V, O> Mapper<C, U, V, O> directMapper(final Mapper.DirectSuccessMapper<C, U, V, O> mapper) {
-        return mapper;
+    public static <C, U, V, O> @NonNull Mapper<C, U, V, O> directMapper(
+            final Mapper.@NonNull DirectSuccessMapper<C, U, V, O> mapper) {
+        return requireNonNull(mapper, "mapper");
     }
 
     public interface Mapper<C, U, V, O> {
diff --git a/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserTripletBuilder.java b/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserTripletBuilder.java
index 6ed47b06d..8e8969988 100644
--- a/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserTripletBuilder.java
+++ b/cloud-core/src/main/java/org/incendo/cloud/parser/aggregate/AggregateParserTripletBuilder.java
@@ -32,6 +32,8 @@
 import org.incendo.cloud.parser.ArgumentParseResult;
 import org.incendo.cloud.type.tuple.Triplet;
 
+import static java.util.Objects.requireNonNull;
+
 public final class AggregateParserTripletBuilder<C, U, V, Z, O> {
 
     /**
@@ -134,8 +136,9 @@ public AggregateParser<C, O> build() {
      * @param <O>    output type
      * @return mapper
      */
-    static <C, U, V, Z, O> Mapper<C, U, V, Z, O> directMapper(final Mapper.DirectSuccessMapper<C, U, V, Z, O> mapper) {
-        return mapper;
+    public static <C, U, V, Z, O> @NonNull Mapper<C, U, V, Z, O> directMapper(
+            final Mapper.@NonNull DirectSuccessMapper<C, U, V, Z, O> mapper) {
+        return requireNonNull(mapper, "mapper");
     }
 
     public interface Mapper<C, U, V, Z, O> {