diff --git a/Crypto/Hash/Blake2.hs b/Crypto/Hash/Blake2.hs
index b957fa3..d86a0d5 100644
--- a/Crypto/Hash/Blake2.hs
+++ b/Crypto/Hash/Blake2.hs
@@ -60,7 +60,7 @@ import           Crypto.Internal.Nat
 data Blake2s (bitlen :: Nat) = Blake2s
   deriving (Show, Typeable)
 
-instance (IsDivisibleBy8 bitlen, KnownNat bitlen)
+instance (IsDivisibleBy8 bitlen, KnownNat bitlen, IsAtLeast bitlen 8, IsAtMost bitlen 256)
       => HashAlgorithm (Blake2s bitlen)
       where
     hashBlockSize  _          = 64
@@ -92,7 +92,7 @@ foreign import ccall unsafe "cryptonite_blake2s_finalize"
 data Blake2b (bitlen :: Nat) = Blake2b
   deriving (Show, Typeable)
 
-instance (IsDivisibleBy8 bitlen, KnownNat bitlen)
+instance (IsDivisibleBy8 bitlen, KnownNat bitlen, IsAtLeast bitlen 8, IsAtMost bitlen 512)
       => HashAlgorithm (Blake2b bitlen)
       where
     hashBlockSize  _          = 128
@@ -112,7 +112,7 @@ foreign import ccall unsafe "cryptonite_blake2b_finalize"
 data Blake2sp (bitlen :: Nat) = Blake2sp
   deriving (Show, Typeable)
 
-instance (IsDivisibleBy8 bitlen, KnownNat bitlen)
+instance (IsDivisibleBy8 bitlen, KnownNat bitlen, IsAtLeast bitlen 8, IsAtMost bitlen 256)
       => HashAlgorithm (Blake2sp bitlen)
       where
     hashBlockSize  _          = 64
@@ -132,7 +132,7 @@ foreign import ccall unsafe "cryptonite_blake2sp_finalize"
 data Blake2bp (bitlen :: Nat) = Blake2bp
   deriving (Show, Typeable)
 
-instance (IsDivisibleBy8 bitlen, KnownNat bitlen)
+instance (IsDivisibleBy8 bitlen, KnownNat bitlen, IsAtLeast bitlen 8, IsAtMost bitlen 512)
       => HashAlgorithm (Blake2bp bitlen)
       where
     hashBlockSize  _          = 128