diff --git a/src/PIL/GdImageFile.py b/src/PIL/GdImageFile.py
index d84876eb6b6..7bb4736af13 100644
--- a/src/PIL/GdImageFile.py
+++ b/src/PIL/GdImageFile.py
@@ -27,6 +27,8 @@ class is not registered for use with :py:func:`PIL.Image.open()`.  To open a
 """
 from __future__ import annotations
 
+from io import BytesIO
+
 from . import ImageFile, ImagePalette, UnidentifiedImageError
 from ._binary import i16be as i16
 from ._binary import i32be as i32
@@ -43,8 +45,10 @@ class GdImageFile(ImageFile.ImageFile):
     format = "GD"
     format_description = "GD uncompressed images"
 
-    def _open(self):
+    def _open(self) -> None:
         # Header
+        assert self.fp is not None
+
         s = self.fp.read(1037)
 
         if i16(s) not in [65534, 65535]:
@@ -76,7 +80,7 @@ def _open(self):
         ]
 
 
-def open(fp, mode="r"):
+def open(fp: BytesIO, mode: str = "r") -> GdImageFile:
     """
     Load texture from a GD image file.