diff --git a/src/PIL/FitsImagePlugin.py b/src/PIL/FitsImagePlugin.py
index 7dce2d60f..e69890bab 100644
--- a/src/PIL/FitsImagePlugin.py
+++ b/src/PIL/FitsImagePlugin.py
@@ -15,7 +15,7 @@ import math
 from . import Image, ImageFile
 
 
-def _accept(prefix):
+def _accept(prefix: bytes) -> bool:
     return prefix[:6] == b"SIMPLE"
 
 
@@ -23,8 +23,10 @@ class FitsImageFile(ImageFile.ImageFile):
     format = "FITS"
     format_description = "FITS"
 
-    def _open(self):
-        headers = {}
+    def _open(self) -> None:
+        assert self.fp is not None
+
+        headers: dict[bytes, bytes] = {}
         while True:
             header = self.fp.read(80)
             if not header:
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 5ab27c359..c1f89af46 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -3486,7 +3486,7 @@ def register_extension(id, extension) -> None:
     EXTENSION[extension.lower()] = id.upper()
 
 
-def register_extensions(id, extensions):
+def register_extensions(id, extensions) -> None:
     """
     Registers image extensions.  This function should not be
     used in application code.