diff --git a/pypdf/_page.py b/pypdf/_page.py
index ced22cce0..3409dc16f 100644
--- a/pypdf/_page.py
+++ b/pypdf/_page.py
@@ -1083,6 +1083,7 @@ def _merge_page(
                 return self._merge_page_writer(
                     page2, page2transformation, ctm, over, expand
                 )
+                return None
         except (AssertionError, AttributeError):
             pass
 
@@ -1175,6 +1176,7 @@ def _merge_page(
         self.replace_contents(ContentStream(new_content_array, self.pdf))
         self[NameObject(PG.RESOURCES)] = new_resources
         self[NameObject(PG.ANNOTS)] = new_annots
+        return None
 
     def _merge_page_writer(
         self,
diff --git a/pypdf/_writer.py b/pypdf/_writer.py
index 6a4fe4324..fb5c6c650 100644
--- a/pypdf/_writer.py
+++ b/pypdf/_writer.py
@@ -2104,6 +2104,7 @@ def clean_forms(
 
             clean(content, images, forms, text_filters)
             page.replace_contents(content)
+        return [], []  # type: ignore[return-value]
 
     def remove_images(
         self,
@@ -3080,6 +3081,7 @@ def find_outline_item(
                 o = cast(TreeObject, o["/Next"])
             else:
                 return None
+        raise PyPdfError("This line is theoretically unreachable.")  # pragma: no cover
 
     def reset_translation(
         self, reader: Union[None, PdfReader, IndirectObject] = None
diff --git a/pyproject.toml b/pyproject.toml
index f0ba519cf..2d44cc222 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -179,7 +179,6 @@ ignore = [
     "PT014",   # Ruff bug: Duplicate of test case at index 1 in `@pytest_mark.parametrize`
     "PTH123",  # `open()` should be replaced by `Path.open()`
     "PYI042",  # Type alias `mode_str_type` should be CamelCase
-    "RET503",  # Missing explicit `return` at the end of function able to return non-`None` value
     "RUF001",  # Detect confusable Unicode-to-Unicode units. Introduces bugs
     "RUF002",  # Detect confusable Unicode-to-Unicode units. Introduces bugs
     "S101",    # Use of `assert` detected
diff --git a/tests/test_merger.py b/tests/test_merger.py
index 183766df9..1a8af7d0a 100644
--- a/tests/test_merger.py
+++ b/tests/test_merger.py
@@ -25,7 +25,6 @@ def merger_operate(merger):
     pdf_forms = RESOURCE_ROOT / "pdflatex-forms.pdf"
     pdf_pw = RESOURCE_ROOT / "libreoffice-writer-password.pdf"
 
-    # string path:
     merger.append(pdf_path)
     merger.append(outline)
     merger.append(pdf_path, pages=pypdf.pagerange.PageRange(slice(0, 0)))