diff --git a/fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/ForEachTaskBuilder.java b/fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/ForEachTaskBuilder.java
index 196ed888e..21ef0e9bc 100644
--- a/fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/ForEachTaskBuilder.java
+++ b/fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/ForEachTaskBuilder.java
@@ -21,6 +21,7 @@
 import io.serverlessworkflow.api.types.TaskItem;
 import io.serverlessworkflow.fluent.spec.spi.ForEachTaskFluent;
 import java.util.List;
+import java.util.Objects;
 import java.util.function.Consumer;
 
 public class ForEachTaskBuilder<T extends BaseTaskItemListBuilder<T>>
@@ -85,6 +86,8 @@ public ForEachTaskBuilder<T> tasks(Consumer<T> doBuilderConsumer) {
 
   public ForTask build() {
     this.forTask.setFor(this.forTaskConfiguration);
+    Objects.requireNonNull(
+        this.forTask.getFor().getIn(), "'in' is a required property for ForTask");
     return this.forTask;
   }
 }
diff --git a/fluent/spec/src/test/java/io/serverlessworkflow/fluent/spec/WorkflowBuilderTest.java b/fluent/spec/src/test/java/io/serverlessworkflow/fluent/spec/WorkflowBuilderTest.java
index 4ba41c203..57af6a590 100644
--- a/fluent/spec/src/test/java/io/serverlessworkflow/fluent/spec/WorkflowBuilderTest.java
+++ b/fluent/spec/src/test/java/io/serverlessworkflow/fluent/spec/WorkflowBuilderTest.java
@@ -119,7 +119,7 @@ void testDoTaskSetAndForEach() {
             .tasks(
                 d ->
                     d.set("initCtx", "$.foo = 'bar'")
-                        .forEach("item", f -> f.each("item").at("$.list")))
+                        .forEach("item", f -> f.each("item").at("index").in("$.list")))
             .build();
 
     List<TaskItem> items = wf.getDo();
diff --git a/impl/test/src/test/java/io/serverlessworkflow/impl/test/ForTaskMissingInTest.java b/impl/test/src/test/java/io/serverlessworkflow/impl/test/ForTaskMissingInTest.java
new file mode 100644
index 000000000..88c3a7a9b
--- /dev/null
+++ b/impl/test/src/test/java/io/serverlessworkflow/impl/test/ForTaskMissingInTest.java
@@ -0,0 +1,46 @@
+/*
+ * Copyright 2020-Present The Serverless Workflow Specification Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.serverlessworkflow.impl.test;
+
+import static io.serverlessworkflow.fluent.spec.dsl.DSL.*;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import io.serverlessworkflow.api.types.Workflow;
+import io.serverlessworkflow.fluent.spec.WorkflowBuilder;
+import org.junit.jupiter.api.Test;
+
+class ForTaskMissingInTest {
+
+  @Test
+  void forTaskWithoutInShouldFailWithDescriptiveMessage() {
+
+    assertThatThrownBy(
+            () -> {
+              Workflow ignored =
+                  WorkflowBuilder.workflow("for-missing-in", "test", "0.1.0")
+                      .tasks(
+                          doTasks(
+                              forEach(
+                                  "loopWithoutIn",
+                                  f ->
+                                      f.each("item")
+                                          .tasks(t -> t.set("noop", s -> s.put("done", true))))))
+                      .build();
+            })
+        .isInstanceOf(NullPointerException.class)
+        .hasMessageContaining("'in' is a required property for ForTask");
+  }
+}