diff --git a/src/field/crypto_bigint_const_monty.rs b/src/field/crypto_bigint_const_monty.rs
index 4e1c25f..4f9526d 100644
--- a/src/field/crypto_bigint_const_monty.rs
+++ b/src/field/crypto_bigint_const_monty.rs
@@ -187,8 +187,12 @@ impl<Mod: Params<LIMBS>, const LIMBS: usize> Neg for ConstMontyField<Mod, LIMBS>
     type Output = Self;
 
     #[inline(always)]
-    fn neg(self) -> Self::Output {
-        Self(self.0.neg())
+    fn neg(mut self) -> Self::Output {
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .neg_mod(Mod::PARAMS.modulus().as_nz_ref());
+        self
     }
 }
 
@@ -291,14 +295,20 @@ impl_op_assign_boilerplate!(DivAssign, div_assign);
 impl<Mod: Params<LIMBS>, const LIMBS: usize> AddAssign<&Self> for ConstMontyField<Mod, LIMBS> {
     #[inline(always)]
     fn add_assign(&mut self, rhs: &Self) {
-        self.0.add_assign(&rhs.0);
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .add_mod(rhs.0.as_montgomery(), Mod::PARAMS.modulus().as_nz_ref());
     }
 }
 
 impl<Mod: Params<LIMBS>, const LIMBS: usize> SubAssign<&Self> for ConstMontyField<Mod, LIMBS> {
     #[inline(always)]
     fn sub_assign(&mut self, rhs: &Self) {
-        self.0.sub_assign(&rhs.0);
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .sub_mod(rhs.0.as_montgomery(), Mod::PARAMS.modulus().as_nz_ref());
     }
 }
 
diff --git a/src/field/crypto_bigint_monty.rs b/src/field/crypto_bigint_monty.rs
index c2287f7..7f05e10 100644
--- a/src/field/crypto_bigint_monty.rs
+++ b/src/field/crypto_bigint_monty.rs
@@ -133,8 +133,12 @@ impl<const LIMBS: usize> Hash for MontyField<LIMBS> {
 impl<const LIMBS: usize> Neg for MontyField<LIMBS> {
     type Output = Self;
 
-    fn neg(self) -> Self::Output {
-        Self(self.0.neg())
+    fn neg(mut self) -> Self::Output {
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .neg_mod(self.0.params().modulus().as_nz_ref());
+        self
     }
 }
 
@@ -243,14 +247,20 @@ impl_op_assign_boilerplate!(DivAssign, div_assign);
 impl<const LIMBS: usize> AddAssign<&Self> for MontyField<LIMBS> {
     #[inline(always)]
     fn add_assign(&mut self, rhs: &Self) {
-        self.0.add_assign(&rhs.0);
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .add_mod(rhs.0.as_montgomery(), self.0.params().modulus().as_nz_ref());
     }
 }
 
 impl<const LIMBS: usize> SubAssign<&Self> for MontyField<LIMBS> {
     #[inline(always)]
     fn sub_assign(&mut self, rhs: &Self) {
-        self.0.sub_assign(&rhs.0);
+        *self.0.as_montgomery_mut() = self
+            .0
+            .as_montgomery()
+            .sub_mod(rhs.0.as_montgomery(), self.0.params().modulus().as_nz_ref());
     }
 }