diff --git a/src/Primitive/Defs.purs b/src/Primitive/Defs.purs
index 9f4c2cd5a..6a413ea04 100644
--- a/src/Primitive/Defs.purs
+++ b/src/Primitive/Defs.purs
@@ -130,8 +130,8 @@ matrixLookup = mkExists $ ForeignOp' { arity: 2, op': op, op: fwd, op_bwd: bwd }
    fwd _ = throw "Matrix and pair of integers expected"
 
    bwd :: OpBwd (Raw MatrixRep × (Int × Int))
-   bwd (((MatrixRep (vss × (i' × _) × (j' × _))) × (i × j)) × v) =
-      Matrix bot (matrixPut i j (const v) (MatrixRep (((<$>) botOf <$> vss) × (i' × bot) × (j' × bot))))
+   bwd ((r × (i × j)) × v) =
+      Matrix bot (matrixPut i j (const v) (botOf r))
          : Constr bot cPair (Int bot i : Int bot j : Nil)
          : Nil