diff --git a/pypdf/_writer.py b/pypdf/_writer.py
index cf50ca0bf..904cdd829 100644
--- a/pypdf/_writer.py
+++ b/pypdf/_writer.py
@@ -203,7 +203,7 @@ 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,
@@ -590,7 +590,7 @@ 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):
@@ -655,7 +655,7 @@ 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
@@ -672,10 +672,12 @@ 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 97acb2a90..b186d4cee 100644
--- a/tests/test_writer.py
+++ b/tests/test_writer.py
@@ -236,6 +236,10 @@ 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()