diff --git a/luigi/worker.py b/luigi/worker.py
index c3ea777b8a..a11f808ae5 100644
--- a/luigi/worker.py
+++ b/luigi/worker.py
@@ -55,7 +55,7 @@
 from luigi.scheduler import DISABLED, DONE, FAILED, PENDING, UNKNOWN, Scheduler, RetryPolicy
 from luigi.scheduler import WORKER_STATE_ACTIVE, WORKER_STATE_DISABLED
 from luigi.target import Target
-from luigi.task import Task, Config, DynamicRequirements
+from luigi.task import Task, Config, DynamicRequirements, flatten
 from luigi.task_register import TaskClassException
 from luigi.task_status import RUNNING
 from luigi.parameter import BoolParameter, FloatParameter, IntParameter, OptionalParameter, Parameter, TimeDeltaParameter
@@ -185,7 +185,7 @@ def run(self):
                 missing = []
                 for dep in self.task.deps():
                     if not self.check_complete(dep):
-                        nonexistent_outputs = [output for output in dep.output() if not output.exists()]
+                        nonexistent_outputs = [output for output in flatten(dep.output()) if not output.exists()]
                         if nonexistent_outputs:
                             missing.append(f'{dep.task_id} ({", ".join(map(str, nonexistent_outputs))})')
                         else:
diff --git a/test/worker_task_test.py b/test/worker_task_test.py
index af51defe6c..d58f740ae3 100644
--- a/test/worker_task_test.py
+++ b/test/worker_task_test.py
@@ -26,6 +26,7 @@
 import luigi
 import luigi.date_interval
 import luigi.notifications
+from luigi.mock import MockTarget
 from luigi.worker import TaskException, TaskProcess
 from luigi.scheduler import DONE, FAILED
 
@@ -106,6 +107,42 @@ def complete(self):
                 None
             ))
 
+    def test_fail_on_unfulfilled_dependencies(self):
+        class NeverCompleteTask(luigi.Task):
+            def complete(self):
+                return False
+
+        class A(NeverCompleteTask):
+            def output(self):
+                return []
+
+        class B(NeverCompleteTask):
+            def output(self):
+                return MockTarget("foo-B")
+
+        class C(NeverCompleteTask):
+            def output(self):
+                return [MockTarget("foo-C1"), MockTarget("foo-C2")]
+
+        class Main(NeverCompleteTask):
+            def requires(self):
+                return [A(), B(), C()]
+
+        task = Main()
+        result_queue = multiprocessing.Queue()
+        task_process = TaskProcess(task, 1, result_queue, mock.Mock())
+
+        with mock.patch.object(result_queue, 'put') as mock_put:
+            task_process.run()
+            expected_missing = [A().task_id, f"{B().task_id} (foo-B)", f"{C().task_id} (foo-C1, foo-C2)"]
+            mock_put.assert_called_once_with((
+                task.task_id,
+                FAILED,
+                StringContaining(f"Unfulfilled dependencies at run time: {', '.join(expected_missing)}"),
+                expected_missing,
+                [],
+            ))
+
     def test_cleanup_children_on_terminate(self):
         """
         Subprocesses spawned by tasks should be terminated on terminate
diff --git a/tox.ini b/tox.ini
index 4208e98eca..12a23a93e2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -26,7 +26,7 @@ deps =
     pytest<7.0
     pytest-cov>=2.0,<3.0
     mock<2.0
-    moto>=1.3.10
+    moto>=1.3.10,<5.0
     HTTPretty==0.8.10
     docker>=2.1.0
     boto>=2.42,<3.0