diff --git a/cborg/cborg.cabal b/cborg/cborg.cabal
index 16bcb1d..39a2a4e 100644
--- a/cborg/cborg.cabal
+++ b/cborg/cborg.cabal
@@ -94,6 +94,7 @@ library
     bytestring              >= 0.10.4  && < 0.12,
     containers              >= 0.5     && < 0.7,
     deepseq                 >= 1.0     && < 1.5,
+    dupIO,
     ghc-prim                >= 0.3.1.0 && < 0.11,
     half                    >= 0.2.2.3 && < 0.4,
     primitive               >= 0.5     && < 0.9,
diff --git a/cborg/src/Codec/CBOR/Write.hs b/cborg/src/Codec/CBOR/Write.hs
index 977685e..20d3139 100644
--- a/cborg/src/Codec/CBOR/Write.hs
+++ b/cborg/src/Codec/CBOR/Write.hs
@@ -75,6 +75,8 @@ import qualified Codec.CBOR.ByteArray.Sliced           as BAS
 import           Codec.CBOR.Encoding
 import           Codec.CBOR.Magic
 
+import qualified Data.Dup                              as Dup
+
 --------------------------------------------------------------------------------
 
 -- | Turn an 'Encoding' into a lazy 'L.ByteString' in CBOR binary
@@ -110,7 +112,9 @@ buildStep vs1 k (BI.BufferRange op0 ope0) =
     go vs1 op0
   where
     go vs !op
-      | op `plusPtr` bound <= ope0 = case vs of
+      | op `plusPtr` bound <= ope0 = do
+        dup_vs <- Dup.dupIO vs
+        case dup_vs of
           TkWord     x vs' -> PI.runB wordMP     x op >>= go vs'
           TkWord64   x vs' -> PI.runB word64MP   x op >>= go vs'