diff --git a/pypdf/_writer.py b/pypdf/_writer.py
index 2a0229935..18f5aaaed 100644
--- a/pypdf/_writer.py
+++ b/pypdf/_writer.py
@@ -202,7 +202,7 @@ def __init__(
 
         self._info_obj: Optional[PdfObject]
         """The PDF files's document information dictionary,
-        the Info entry in the PDF file's trailer dictionary."""
+        defined by Info in the PDF file's trailer dictionary."""
 
         self._ID: Union[ArrayObject, None] = None
         """The PDF file identifier,
@@ -618,7 +618,7 @@ def insert_page(
         """
         assert self.flattened_pages is not None, "mypy"
         if index < 0:
-            index = len(self.flattened_pages) + index
+            index += len(self.flattened_pages)
         if index < 0:
             raise ValueError("Invalid index value")
         if index >= len(self.flattened_pages):
@@ -683,7 +683,7 @@ def insert_blank_page(
         """
         Insert a blank page to this PDF file and return it.
 
-        If no page size is specified, use the size of the last page.
+        If no page size is specified for a dimension, use the size of the last page.
 
         Args:
             width: The width of the new page expressed in default user
@@ -700,10 +700,12 @@ def insert_blank_page(
                 and previous page does not exist.
 
         """
-        if width is None or (height is None and index < self.get_num_pages()):
+        if (width is None or height is None) and index < self.get_num_pages():
             oldpage = self.pages[index]
-            width = oldpage.mediabox.width
-            height = oldpage.mediabox.height
+            if width is None:
+                width = oldpage.mediabox.width
+            if height is None:
+                height = oldpage.mediabox.height
         page = PageObject.create_blank_page(self, width, height)
         self.insert_page(page, index)
         return page
diff --git a/tests/test_writer.py b/tests/test_writer.py
index a7ddb4748..ac21627f0 100644
--- a/tests/test_writer.py
+++ b/tests/test_writer.py
@@ -238,6 +238,10 @@ def writer_operate(writer: PdfWriter) -> None:
     writer.page_mode = NameObject("/UseOC")
     assert writer._get_page_mode() == "/UseOC"
     writer.insert_blank_page(width=100, height=100)
+    page = writer.insert_blank_page(width=100)
+    assert page.mediabox.height == 100
+    page = writer.insert_blank_page(height=100)
+    assert page.mediabox.width == 100
     writer.insert_blank_page()  # without parameters
 
     writer.remove_images()
@@ -255,6 +259,25 @@ def writer_operate(writer: PdfWriter) -> None:
         assert k in objects_hash, f"Missing {v}"
 
 
+def test_insert_blank_page():
+    writer = PdfWriter(clone_from=RESOURCE_ROOT / "crazyones.pdf")
+
+    old_page = writer.pages[0]
+    page = writer.insert_blank_page(index=0)
+    assert page.mediabox.width == old_page.mediabox.width
+    assert page.mediabox.height == old_page.mediabox.height
+
+    old_page = writer.pages[0]
+    page = writer.insert_blank_page(width=50, index=0)
+    assert page.mediabox.width == 50
+    assert page.mediabox.height == old_page.mediabox.height
+
+    old_page = writer.pages[0]
+    page = writer.insert_blank_page(width=10, height=20, index=0)
+    assert page.mediabox.width == 10
+    assert page.mediabox.height == 20
+
+
 @pytest.mark.parametrize(
     ("convert", "needs_cleanup"),
     [