diff --git a/src/Data/Text/Internal/IsAscii.hs b/src/Data/Text/Internal/IsAscii.hs
index c848e54f..761f08b5 100644
--- a/src/Data/Text/Internal/IsAscii.hs
+++ b/src/Data/Text/Internal/IsAscii.hs
@@ -13,6 +13,7 @@ module Data.Text.Internal.IsAscii where
 import Prelude (Bool(..), Int)
 import qualified Data.Char as Char
 import {-# SOURCE #-} Data.Text (all)
+import qualified Data.ByteString as BS
 #else
 import Data.Text.Internal.ByteStringCompat (withBS)
 import Data.Text.Internal.Unsafe (unsafeWithForeignPtr)
@@ -64,7 +65,7 @@ isAscii (Text (A.ByteArray arr) off len) =
 -- | Length of the longest ASCII prefix.
 asciiPrefixLength :: ByteString -> Int
 #if defined(PURE_HASKELL)
-asciiPrefixLength = P.const 0
+asciiPrefixLength = BS.length P.. BS.takeWhile (P.< 0x80)
 #else
 asciiPrefixLength bs = unsafeDupablePerformIO $ withBS bs $ \ fp len ->
   unsafeWithForeignPtr fp $ \src -> do