diff --git a/Data/Monoid/Instances/ByteString/UTF8.hs b/Data/Monoid/Instances/ByteString/UTF8.hs
--- a/Data/Monoid/Instances/ByteString/UTF8.hs
+++ b/Data/Monoid/Instances/ByteString/UTF8.hs
@@ -1,5 +1,5 @@
 {- 
-    Copyright 2013-2015 Mario Blazevic
+    Copyright 2013-2016 Mario Blazevic
 
     License: BSD3 (see BSD3-LICENSE.txt file)
 -}
@@ -218,7 +218,8 @@
                                of Just s1 -> dropASCII s1 tl
                                   Nothing -> (bs, s)
    {-# INLINE spanMaybe #-}
-   spanMaybe' s0 f (ByteStringUTF8 bs0) = (ByteStringUTF8 $ ByteString.take (ByteString.length bs0 - ByteString.length dropped) bs0,
+   spanMaybe' s0 f (ByteStringUTF8 bs0) = (ByteStringUTF8 $
+                                           ByteString.take (ByteString.length bs0 - ByteString.length dropped) bs0,
                                            ByteStringUTF8 dropped,
                                            s')
       where (dropped, s') = dropASCII s0 bs0
@@ -373,19 +374,19 @@
 reverseBytesToChar :: (ByteString -> a) -> (Char -> a) -> [Word8] -> a
 reverseBytesToChar ft fc [w] = if w < 0x80 then fc (w2c w) else ft (ByteString.singleton w)
 reverseBytesToChar ft fc [b0, b1] =
-  assert (0x80 <= b0 && b0 < 0xC0 && 0xC0 <= b1) $
+  assert (0x80 <= b0 && b0 < 0xC0) $
   if 0xC2 <= b1 && b1 < 0xE0
   then fc (chr (shiftL (fromIntegral b1 .&. 0x1F) 6 .|. fromIntegral b0 .&. 0x3F))
   else ft (ByteString.pack [b1, b0])
 reverseBytesToChar ft fc [b0, b1, b2] =
-  assert (0x80 <= b0 && b0 < 0xC0 && 0x80 <= b1 && b1 < 0xC0 && 0xC0 <= b2) $
+  assert (0x80 <= b0 && b0 < 0xC0 && 0x80 <= b1 && b1 < 0xC0) $
   if (0xE0 < b2 || 0xE0 == b2 && 0xA0 <= b1) && b2 < 0xF0
   then fc (chr (shiftL (fromIntegral b2 .&. 0xF) 12
                 .|. shiftL (fromIntegral b1 .&. 0x3F) 6
                 .|. fromIntegral b0 .&. 0x3F))
   else ft (ByteString.pack [b2, b1, b0])
 reverseBytesToChar ft fc [b0, b1, b2, b3] =
-  assert (0x80 <= b0 && b0 < 0xC0 && 0x80 <= b1 && b1 < 0xC0 && 0x80 <= b2 && b2 < 0xC0 && 0xC0 <= b3) $
+  assert (0x80 <= b0 && b0 < 0xC0 && 0x80 <= b1 && b1 < 0xC0 && 0x80 <= b2 && b2 < 0xC0) $
   if (0xF0 < b3 || 0xF0 == b3 && 0x90 <= b2) && b3 < 0xF4
   then fc (chr (shiftL (fromIntegral b3 .&. 0x7) 18
                 .|. shiftL (fromIntegral b2 .&. 0x3F) 12
@@ -484,8 +485,9 @@
 
 charStartIndex :: Int -> ByteString -> Int
 charStartIndex n _ | n <= 0 = 0
-charStartIndex n bs =
-   case List.drop (pred n) (ByteString.findIndices byteStartsCharacter $ ByteString.drop 1 bs)
-   of [] -> ByteString.length bs
-      k:_ -> succ k
+charStartIndex n0 bs = ByteString.foldr count (const $ ByteString.length bs) bs (n0, False, 0)
+      where count byte _    (0, high, i) | byte < 0x80 || byte >= 0xC0 || not high = i
+            count byte cont (n, high, i) | byte < 0x80 = cont (pred n, False, succ i)
+                                         | byte < 0xC0 = cont (if high then n else pred n, True, succ i)
+                                         | otherwise = cont (pred n, True, succ i)
 {-# INLINE charStartIndex #-}
diff --git a/Test/TestMonoidSubclasses.hs b/Test/TestMonoidSubclasses.hs
--- a/Test/TestMonoidSubclasses.hs
+++ b/Test/TestMonoidSubclasses.hs
@@ -783,9 +783,6 @@
 instance Arbitrary a => Arbitrary (Sum a) where
    arbitrary = fmap Sum arbitrary
 
-instance Arbitrary a => Arbitrary (Vector a) where
-   arbitrary = fmap fromList arbitrary
-
 instance Arbitrary ByteStringUTF8 where
    arbitrary = fmap ByteStringUTF8 arbitrary
 
@@ -824,9 +821,6 @@
 
 instance CoArbitrary a => CoArbitrary (Sum a) where
    coarbitrary (Sum a) = coarbitrary a
-
-instance CoArbitrary a => CoArbitrary (Vector a) where
-   coarbitrary = coarbitrary . toList
 
 instance CoArbitrary ByteStringUTF8 where
    coarbitrary (ByteStringUTF8 bs) = coarbitrary bs
diff --git a/monoid-subclasses.cabal b/monoid-subclasses.cabal
--- a/monoid-subclasses.cabal
+++ b/monoid-subclasses.cabal
@@ -1,5 +1,5 @@
 Name:                monoid-subclasses
-Version:             0.4.1.2
+Version:             0.4.2
 Cabal-Version:       >= 1.10
 Build-Type:          Simple
 Synopsis:            Subclasses of Monoid
@@ -35,7 +35,7 @@
   Build-Depends:     base >= 4.5 && < 5,
                      bytestring >= 0.9 && < 1.0, containers >= 0.5.2.0 && < 0.6, text >= 0.11 && < 1.3,
                      vector >= 0.9 && < 0.12, primes == 0.2.*,
-                     QuickCheck == 2.*, quickcheck-instances == 0.3.*, tasty >= 0.7, tasty-quickcheck >= 0.7,
+                     QuickCheck == 2.*, quickcheck-instances >= 0.3.12 && <0.4, tasty >= 0.7, tasty-quickcheck >= 0.7,
                      monoid-subclasses
   Main-is:           Test/TestMonoidSubclasses.hs
   default-language:  Haskell2010