diff --git a/src/gadgets/biguint.rs b/src/gadgets/biguint.rs
index 1ccf9c3a..2c34d044 100644
--- a/src/gadgets/biguint.rs
+++ b/src/gadgets/biguint.rs
@@ -52,7 +52,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         }
     }
 
-    pub fn pad_biguints<'a>(
+    pub fn pad_biguints(
         &mut self,
         a: BigUintTarget,
         b: BigUintTarget,
@@ -141,12 +141,11 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
     }
 
     pub fn mul_biguint(&mut self, a: &BigUintTarget, b: &BigUintTarget) -> BigUintTarget {
-        let num_limbs = a.limbs.len();
-        debug_assert!(b.limbs.len() == num_limbs);
+        let total_limbs = a.limbs.len() + b.limbs.len();
 
-        let mut to_add = vec![vec![]; 2 * num_limbs];
-        for i in 0..num_limbs {
-            for j in 0..num_limbs {
+        let mut to_add = vec![vec![]; total_limbs];
+        for i in 0..a.limbs.len() {
+            for j in 0..b.limbs.len() {
                 let (product, carry) = self.mul_u32(a.limbs[i], b.limbs[j]);
                 to_add[i + j].push(product);
                 to_add[i + j + 1].push(carry);
@@ -155,7 +154,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
 
         let mut combined_limbs = vec![];
         let mut carry = self.zero_u32();
-        for i in 0..2 * num_limbs {
+        for i in 0..total_limbs {
             to_add[i].push(carry);
             let (new_result, new_carry) = self.add_many_u32(to_add[i].clone());
             combined_limbs.push(new_result);
@@ -243,7 +242,7 @@ mod tests {
     use num::{BigUint, FromPrimitive, Integer};
 
     use crate::{
-        field::{crandall_field::CrandallField, field_types::PrimeField},
+        field::crandall_field::CrandallField,
         iop::witness::PartialWitness,
         plonk::{circuit_builder::CircuitBuilder, circuit_data::CircuitConfig, verifier::verify},
     };
diff --git a/src/gadgets/nonnative.rs b/src/gadgets/nonnative.rs
index 16a62776..c8539629 100644
--- a/src/gadgets/nonnative.rs
+++ b/src/gadgets/nonnative.rs
@@ -21,12 +21,44 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             .collect()
     }
 
+    pub fn biguint_to_ff<FF: Field>(&mut self, x: &BigUintTarget) -> ForeignFieldTarget<FF> {
+        ForeignFieldTarget {
+            limbs: x.limbs.clone(),
+            _phantom: PhantomData,
+        }
+    }
+
     pub fn ff_to_biguint<FF: Field>(&mut self, x: &ForeignFieldTarget<FF>) -> BigUintTarget {
         BigUintTarget {
             limbs: x.limbs.clone(),
         }
     }
 
+    pub fn constant_ff<FF: Field>(&mut self, x: FF) -> ForeignFieldTarget<FF> {
+        let x_biguint = self.constant_biguint(&x.to_biguint());
+        self.biguint_to_ff(&x_biguint)
+    }
+
+    // Assert that two ForeignFieldTarget's, both assumed to be in reduced form, are equal.
+    pub fn connect_ff_reduced<FF: Field>(
+        &mut self,
+        lhs: &ForeignFieldTarget<FF>,
+        rhs: &ForeignFieldTarget<FF>,
+    ) {
+        let min_limbs = lhs.limbs.len().min(rhs.limbs.len());
+
+        for i in 0..min_limbs {
+            self.connect_u32(lhs.limbs[i], rhs.limbs[i]);
+        }
+
+        for i in min_limbs..lhs.limbs.len() {
+            self.assert_zero_u32(lhs.limbs[i]);
+        }
+        for i in min_limbs..rhs.limbs.len() {
+            self.assert_zero_u32(rhs.limbs[i]);
+        }
+    }
+
     // Add two `ForeignFieldTarget`s.
     pub fn add_nonnative<FF: Field>(
         &mut self,
@@ -82,3 +114,40 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         self.reduce(&x_biguint)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use anyhow::Result;
+
+    use crate::field::crandall_field::CrandallField;
+    use crate::field::field_types::Field;
+    use crate::field::secp256k1::Secp256K1Base;
+    use crate::iop::witness::PartialWitness;
+    use crate::plonk::circuit_builder::CircuitBuilder;
+    use crate::plonk::circuit_data::CircuitConfig;
+    use crate::plonk::verifier::verify;
+
+    #[test]
+    fn test_nonnative_add() -> Result<()> {
+        type FF = Secp256K1Base;
+        let x_ff = FF::rand();
+        let y_ff = FF::rand();
+        let sum_ff = x_ff + y_ff;
+
+        type F = CrandallField;
+        let config = CircuitConfig::large_config();
+        let pw = PartialWitness::new();
+        let mut builder = CircuitBuilder::<F, 4>::new(config);
+
+        let x = builder.constant_ff(x_ff);
+        let y = builder.constant_ff(y_ff);
+        let sum = builder.add_nonnative(&x, &y);
+
+        let sum_expected = builder.constant_ff(sum_ff);
+        builder.connect_ff_reduced(&sum, &sum_expected);
+
+        let data = builder.build();
+        let proof = data.prove(pw).unwrap();
+        verify(proof, &data.verifier_only, &data.common)
+    }
+}