diff --git a/heudiconv/heuristics/convertall_custom.py b/heudiconv/heuristics/convertall_custom.py
index 780ecc76..26f0ca05 100644
--- a/heudiconv/heuristics/convertall_custom.py
+++ b/heudiconv/heuristics/convertall_custom.py
@@ -4,11 +4,18 @@
 behavior of an already existing heuristic without complete rewrite.  Such approach could be
 useful for heuristic like  reproin  to overload mapping etc.
 """
+from __future__ import annotations
+
+from typing import Any
+
+import nibabel.nicom.dicomwrappers as dw
 
 from .convertall import *  # noqa: F403
 
 
-def custom_seqinfo(series_files, wrapper, **kw):  # noqa: U100
+def custom_seqinfo(
+    series_files: list[str], wrapper: dw.Wrapper, **kw: Any  # noqa: U100
+) -> tuple[str | None, str]:
     """Demo for extracting custom header fields into custom_seqinfo field
 
     Operates on already loaded DICOM data.
diff --git a/heudiconv/tests/test_dicoms.py b/heudiconv/tests/test_dicoms.py
index 12ac29c6..4cb28290 100644
--- a/heudiconv/tests/test_dicoms.py
+++ b/heudiconv/tests/test_dicoms.py
@@ -108,7 +108,7 @@ def test_custom_seqinfo() -> None:
 
     seqinfos = group_dicoms_into_seqinfos(
         dcmfiles, "studyUID", flatten=True, custom_seqinfo=custom_seqinfo
-    )
+    )  # type: ignore
 
     seqinfo = list(seqinfos.keys())[0]