diff --git a/src/PIL/FliImagePlugin.py b/src/PIL/FliImagePlugin.py
index ccb8a5953..b9cc8ad6c 100644
--- a/src/PIL/FliImagePlugin.py
+++ b/src/PIL/FliImagePlugin.py
@@ -48,6 +48,7 @@ class FliImageFile(ImageFile.ImageFile):
 
     def _open(self) -> None:
         # HEAD
+        assert self.fp is not None
         s = self.fp.read(128)
         if not (_accept(s) and s[20:22] == b"\x00\x00"):
             msg = "not an FLI/FLC file"
@@ -77,8 +78,7 @@ class FliImageFile(ImageFile.ImageFile):
 
         if i16(s, 4) == 0xF100:
             # prefix chunk; ignore it
-            self.__offset = self.__offset + i32(s)
-            self.fp.seek(self.__offset)
+            self.fp.seek(self.__offset + i32(s))
             s = self.fp.read(16)
 
         if i16(s, 4) == 0xF1FA:
@@ -111,6 +111,7 @@ class FliImageFile(ImageFile.ImageFile):
         # load palette
 
         i = 0
+        assert self.fp is not None
         for e in range(i16(self.fp.read(2))):
             s = self.fp.read(2)
             i = i + s[0]