diff --git a/Crypto/ECC/Simple/Prim.hs b/Crypto/ECC/Simple/Prim.hs
index 4a36b05..fcb0a3c 100644
--- a/Crypto/ECC/Simple/Prim.hs
+++ b/Crypto/ECC/Simple/Prim.hs
@@ -49,7 +49,7 @@ pointNegate :: Curve curve => Point curve -> Point curve
 pointNegate        PointO     = PointO
 pointNegate point@(Point x y) =
     case curveType point of
-        CurvePrime {}  -> Point x (-y)
+        CurvePrime (CurvePrimeParam p) -> Point x (p - y)
         CurveBinary {} -> Point x (x `addF2m` y)
 
 -- | Elliptic Curve point addition.
diff --git a/Crypto/PubKey/ECC/Prim.hs b/Crypto/PubKey/ECC/Prim.hs
index a4c10b5..d87672b 100644
--- a/Crypto/PubKey/ECC/Prim.hs
+++ b/Crypto/PubKey/ECC/Prim.hs
@@ -31,9 +31,9 @@ scalarGenerate curve = generateBetween 1 (n - 1)
 -- | Elliptic Curve point negation:
 -- @pointNegate c p@ returns point @q@ such that @pointAdd c p q == PointO@.
 pointNegate :: Curve -> Point -> Point
-pointNegate _           PointO     = PointO
-pointNegate CurveFP{}  (Point x y) = Point x (-y)
-pointNegate CurveF2m{} (Point x y) = Point x (x `addF2m` y)
+pointNegate _            PointO     = PointO
+pointNegate (CurveFP c) (Point x y) = Point x (ecc_p c - y)
+pointNegate CurveF2m{}  (Point x y) = Point x (x `addF2m` y)
 
 -- | Elliptic Curve point addition.
 --