diff --git a/haystack/core/component/types.py b/haystack/core/component/types.py
index 4c949dad38..db4ddb917c 100644
--- a/haystack/core/component/types.py
+++ b/haystack/core/component/types.py
@@ -97,7 +97,17 @@ def __post_init__(self) -> None:
         # alias for Iterable[int]. Since we're interested in getting the inner type `int`, we call `get_args`
         # twice: the first time to get `list[int]` out of `Variadic`, the second time to get `int` out of `list[int]`.
         if self.is_lazy_variadic or self.is_greedy:
-            self.type = get_args(get_args(self.type)[0])[0]
+            outer_args = get_args(self.type)
+            inner_type = outer_args[0]
+            inner_args = get_args(inner_type)
+            if not inner_args:
+                from haystack.core.errors import ComponentError
+
+                raise ComponentError(
+                    f"Variadic input '{self.name}' must have a type argument, e.g. Variadic[int]. "
+                    f"Got bare {inner_type!r} without a type argument."
+                )
+            self.type = inner_args[0]
 
 
 class InputSocketTypeDescriptor(TypedDict):
diff --git a/releasenotes/notes/fix-variadic-unsubscripted-indexerror-32788f5484bd3b5c.yaml b/releasenotes/notes/fix-variadic-unsubscripted-indexerror-32788f5484bd3b5c.yaml
new file mode 100644
index 0000000000..13c638636f
--- /dev/null
+++ b/releasenotes/notes/fix-variadic-unsubscripted-indexerror-32788f5484bd3b5c.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix ``InputSocket.__post_init__`` crashing with ``IndexError`` when a Variadic input
+    uses a bare ``Iterable`` without a type argument (e.g. ``Annotated[Iterable, HAYSTACK_VARIADIC_ANNOTATION]``).
+    Now raises ``ComponentError`` with a clear message instead of an unhelpful ``IndexError``.
diff --git a/test/core/component/test_sockets.py b/test/core/component/test_sockets.py
index 2892487120..e4fafe91ba 100644
--- a/test/core/component/test_sockets.py
+++ b/test/core/component/test_sockets.py
@@ -85,3 +85,17 @@ def test_contains(self):
         assert "input_1" in io
         assert "input_2" in io
         assert "invalid" not in io
+
+
+def test_input_socket_variadic_without_type_arg_raises_component_error():
+    """Bare Variadic (no type argument) should raise ComponentError, not IndexError."""
+    from collections.abc import Iterable
+    from typing import Annotated
+
+    from haystack.core.component.types import HAYSTACK_VARIADIC_ANNOTATION
+    from haystack.core.errors import ComponentError
+
+    bare_variadic = Annotated[Iterable, HAYSTACK_VARIADIC_ANNOTATION]
+
+    with pytest.raises(ComponentError, match="must have a type argument"):
+        InputSocket("inputs", bare_variadic)