diff --git a/Data/ByteString/Internal/Pure.hs b/Data/ByteString/Internal/Pure.hs
index 59839cfcf..f29c020fc 100644
--- a/Data/ByteString/Internal/Pure.hs
+++ b/Data/ByteString/Internal/Pure.hs
@@ -179,8 +179,10 @@ isValidUtf8' idx !len = go 0
     indexIsCont i =
         -- We use a signed comparison to avoid an extra comparison with 0x80,
         -- since _signed_ 0x80 is -128.
-        let i' = fromIntegral @Word8 @Int8 (indexWord8 i)
-        in i' <= (fromIntegral (0xBF :: Word8))
+        let
+           v :: Int8
+           v = fromIntegral (indexWord8 i)
+        in v <= (fromIntegral (0xBF :: Word8))
 
     go !i
       | i >= len  = pure True -- done