diff --git a/src/cosy_luigi/utils/traversals.py b/src/cosy_luigi/utils/traversals.py
index 6e279ff..1b323c5 100644
--- a/src/cosy_luigi/utils/traversals.py
+++ b/src/cosy_luigi/utils/traversals.py
@@ -4,13 +4,13 @@
 
 import inspect
 from abc import ABC
-from collections.abc import Sequence
+from collections.abc import Iterable, Sequence
 from typing import TYPE_CHECKING, cast
 
 from cosy_luigi import CoSyLuigiTask
 
 if TYPE_CHECKING:
-    from collections.abc import Iterable, Sequence
+    from collections.abc import Sequence
 
 
 def flatten(
@@ -32,8 +32,12 @@ def flatten(
         for task_or_task_collection in heterogeneous_task_collection
         for task in (
             flatten(*cast("Iterable[type[CoSyLuigiTask]]", task_or_task_collection))
-            if isinstance(task_or_task_collection, (tuple, list))
-            else cast("type[CoSyLuigiTask]", task_or_task_collection).get_all_variants()
+            if isinstance(task_or_task_collection, Iterable)
+            else (
+                variant
+                for variant in cast("type[CoSyLuigiTask]", task_or_task_collection).get_all_variants()
+                if not (inspect.isabstract(variant) or ABC in variant.__bases__)
+            )
             if inspect.isabstract(task_or_task_collection)
             or ABC in cast("type[CoSyLuigiTask]", task_or_task_collection).__bases__
             else (task_or_task_collection,)
diff --git a/tests/test_abstract_variant_expansion.py b/tests/test_abstract_variant_expansion.py
index 5d07d2b..4a50315 100644
--- a/tests/test_abstract_variant_expansion.py
+++ b/tests/test_abstract_variant_expansion.py
@@ -17,6 +17,14 @@ class ABCInheritedTask(CoSyLuigiTask, ABC):
     """An abstract class. This class is abstract because it inherits from ABC."""
 
 
+class ABCTaskFromABCInherited(ABCInheritedTask, ABC):
+    """An abstract class that implements ABCInheritedTask."""
+
+
+class DeeperConcreteTaskFromABCTaskFromABCInherited(ABCTaskFromABCInherited):
+    """A class that indirectly implements ABCInheritedTask by inheriting from ABCTaskFromABCInherited."""
+
+
 class ConcreteTaskFromABCInherited(ABCInheritedTask):
     """A class that implements ABCInheritedTask."""
 
@@ -81,7 +89,11 @@ def test_expansion_from_abc():
     """Tests if adding a class that is abstract because it inherits from ABC expands to all of its subclasses when
     added to a CoSyLuigiRepo."""
     repo = CoSyLuigiRepo(ABCInheritedTask)
-    assert repo.luigi_repo == {ConcreteTaskFromABCInherited, DeeperConcreteTaskFromABCInherited}
+    assert repo.luigi_repo == {
+        ConcreteTaskFromABCInherited,
+        DeeperConcreteTaskFromABCInherited,
+        DeeperConcreteTaskFromABCTaskFromABCInherited,
+    }
 
 
 def test_expansion_from_abstract():
@@ -99,6 +111,7 @@ def test_expansion_from_abc_and_abstract():
     repo = CoSyLuigiRepo(ABCInheritedTask, AbstractTask)
     assert repo.luigi_repo == {
         ConcreteTaskFromABCInherited,
+        DeeperConcreteTaskFromABCTaskFromABCInherited,
         DeeperConcreteTaskFromABCInherited,
         ConcreteTaskFromAbstract,
         DeeperConcreteTaskFromAbstract,